I would second Dickinson and also suggest Gettysburg (which is more in the orbit of Washint on DC than you might guess). Totally agree with @Lindagaf about NY schools. I’m not familiar with the programs at Hobart, St. Lawrence, Union, etc, for your interests, but they feel much like New England schools.
I will add these all to his list. He is certainly not against New York. I think he is really thinking East Coast in general, and since we have lots of family near Boston, we tend to center ourselves there. I spent time in Philly for grad school so am totally good with PA schools as well - beautiful places and agree that they feel a lot like New England LACs. I think he would actually be happier somewhere that isn’t as cold as MA in the wintertime - he has NO idea what it feels like to be in cold!!
@emily29, Many of the schools mentioned upthread are included in The Best 382 Colleges.* The Princeton Review.* 2018 edition
Great Schools for Political Science/Government Majors
Amherst
American U
Bates
Bard
Bowdoin
Bryn Mawr
BYU
Carleton
Claremont Mckenna
Clark U
Holy Cross
Columbia
Connecticut College
Davidson
Dickinson
Drew
Franklin and Marshall
George Mason
Georgetown
George Washington
Gettysburg
Gonzaga
Grinnell
Harvard
Kenyon
Macalester
McGill
Princeton
Scripps
Stanford
Swarthmore
Syracuse
UC Berkeley
UCLA
U Washington
Vassar
Wake Forest
Yale
Source:* Princeton Review*
I agree you might want to check out Gettysburg if you get a chance to see Dickinson. They are very close to one another.
And Franklin and Marshall isn’t too far either.
I’m about the 10th person to say so, but Dickinson. 
A bit of a reach for him, but may not be out of reach. Show interest, apply EA if not ED (they have both).
@emily29 Can be get time accommodations for the SAT? Maybe also take the ACT and see which he scores higher on. I would definitely apply ED. And visit. Show them love.
If you’re willing to consider a ~small university (~5800 undergrads) located far outside your NE comfort zone, then check out the University of Denver. It is one of the smallest, least selective universities to show up regularly in Foreign Policy magazine “Ivory Tower” reviews of top IR programs.
https://en.wikipedia.org/wiki/Josef_Korbel_School_of_International_Studies
Denver’s Chinese language offerings appear to be richer than Dickinson’s; the city of Denver is much livelier than Carlisle, PA (or some of the NESCAC towns.) He could root for the Red Sox when they come to Coors Stadium (at his own risk.) Personally, I prefer Boston to Denver, but for a teenager or 20-something, the Denver area has a lot to offer.
If he pulls in a 1300 or less, Dickinson should be in the reach category but would be an excellent choice. For matches, consider Gettysburg, ClarkU, Wheaton and American U. Some of the schools listed here are out of reach IMHO including Davidson, Middlebury, Conn College. Both UVM and UNH are much larger than 5000 and if he really wants something smaller they are not a fit, although they are both in great little college towns. For low matches and possible safeties I would include St. Michaels and University of New England.
GO RED SOX !!
St Lawrence in upstate NY – lots of Boston area kids. Has Chinese and Arabic concentrations, as well as International Relations and, of course, Poli Sci major. There is a D.C. semester as well as the usual study abroad. About 2200 kids, as best I recall, plus great school spirit – it is a D3 school, but hockey is D1 with some great rivalries. We visited several times with my son as a prospective athlete, and the students seemed down to earth, friendly, it felt like a cooperative environment. Great for outdoor and winter sports people, as it sits in the Adirondacks. It could be a little tricky to get in and out of from CA, though school runs buses to Boston and NYC for breaks, so could fly from there.
Has DS worked with GC using Naviance to determine schools that would be a fit based on academic profile? If not, best to look at schools where DS profiles in the mid-50% for SAT - generally schools where a 1300 is in the mid-50% won’t penalize a 3.6 W GPA as much, simply because they are not a selective as a tippy top schools where everyone has a 1400+ SAT and a 4.0 W GPA.
I know you say your S wants a smaller school, but I want to mention that sometimes there are smaller communities within large schools. For example, at UVM there is the living/learning center dorm, with a languages/culture program. If he were to join that, he’d be a part of that small community as a foundation, then you add in the smallness of the Chinese and Arabic concentrations, and there will be cross-over with these groups. All of a sudden, it takes this massive school down to a much more manageable size. Your S would still have to deal with the administration of a large school, though. FYI, they have a Center for Academic Success that handles issues for kids with learning differences, and I wonder whether they help follow up with the kids. They say they are willing to talk with and meet prospective students, so I wonder whether it’s worth a call.
Stadium? George Carlin would roll over in his grave. Baseball is not played in stadiums but in ‘parks’. It’s Coors FIELD. And there are plenty of Red Sox fans in Denver. Remember the Sox won the World Series here in 2007. Unfortunately, many Yankees fans too but we just ignore them.
I agree that DU is a good option for both IR and several languages. One problem with really small (and I consider anything under 2500 really small) schools is that they can’t offer as many classes in one subject or as many languages into the depths. We looked at some schools with only 5 professors in a department. The course catalog had 25 English lit courses listed, but only 3-5 were offered at any one time The one you want may not cycle in for two years and then hope you aren’t on Semester Abroad when it does.
Yes, we have been using Naviance, but not a lot of kids from our high school apply to smaller colleges back East, so less data there from that perspective, but will continue to hone in on the ones suggested here! Thanks!
^ Oops, must be showing my age. Where I grew up, MLB used to be played in a place called Memorial Stadium. I’ll definitely need to be careful next time I’m Denver!
(Or at Oriole Park for that matter.)
Post #33 @emilys29 Geographic diversity in his favor.
^ Speaking for the NY contingent, there’s always Yankee STADIUM. Had to throw that in there after the potshots us NYers had to take 
^^I’m taking bets that he meets a nice Jewish girl from NY or NJ and converts to the Yankees.
I was driving through a Massachusetts town with friends. A police officer had pulled over a car. A friend said he knew why the guy got stopped. He had a Yankees license plate with a Yankees frame around it.
A wild thought that meets some of OP’s criteria is Bishop’s University in Quebec. It is not quite in New England, being about 29 miles north of the border with Vermont. It has a very strong languages program but I think that it is missing Mandarin (it has very good Spanish and French programs, and also has Japanese and German). It also has a good International Studies program and of course has foreign exchange with many universities in many countries. An unweighted 3.5 GPA will be fine, and they won’t care about Freshman year of high school and also are SAT optional (although “1250ish” would probably be worth sending to them). It is small with about 2,400 students. It is also quite affordable. Criticizing the Yankees would be fine, as long as he doesn’t criticize Les Canadiens.