My suggestion is to call the heads of each department at each school and basically ask them why you should choose their school? For Pitzer, you might want to try the IIS (International and Intercultural Studies) Department.
If your son is interested in IR and Japanese, he might want to consider doing Asian Studies. It is a 5C major, offered through Pomona. What that means is, you will take classes at all 5 colleges rather than just one or two (as if you were doing IR, or IIS), but the department is at Pomona. Apparently, in days gone by, Pitzer offered Asian Studies, but when we introduced IIS with the East Asian focus (can also focus on Africa, middle east?, 3rd world countries, and a few others) we got rid of Asian Studies as a major offered at Pitzer. He could also try and do IR through PO, SC, or CMC.
Here is the Asian Studies Dept website: The Asian Studies Program - Pomona College
If your son wanted to combine Japanese and Asian Studies, he could take courses that focus more on Japan, and take IR classes that relate to Japan at CMC or elsewhere.
Another option is to design your own major that combines all of these, but it might be difficult to differentiate it from a regular IR major or a regular Asian Studies major, when the committee is reviewing the special major, and would be easier to do IR or Asian Studies.
I'm not really sure how to compare Pitzer to the other schools mentioned, but I want to say Pitzer might be a better option, just because you have access to the facilities and classes at the other Claremont Colleges.
If you are interested in the Asian mentoring program, Pitzer has APAC, which has a sponsor program where your student will be assigned an upper class sponsor to help you through the ropes, etc. They collaborate with the other Asian mentor programs as well at the other colleges. But you also have a regular mentor, who lives along side you, that is also there to help you and your adjustment to college.
I'd also recommend if you can make it out to all of the admitted students days at all the schools, but being international, I know it can be difficult.
The weather here is a definite plus over the other schools on the East Coast, and there are also research opportunites with professors here, especially if you are doing a science through the Joint Science Dept.
Sorry this may have been kind of all over the place, let me know if you need any clarification! Or have any other questions.