Suggest make the main goal the primary focus. Lest you risk making a secondary avocational interest the driving force and as a result sub-optimize for the main goal…
If your son really wants to be an engineer he should go to a school that itself offers an engineering major. With subspecialties in the areas of engineering that may interest him. (To assess that, suggest look at the courses being offered in the Registrar’s list of courses).
IMO relatively few students who start out thinking about 3-2 programs wind up following through with them.
My D2 said her favorite courses in college were the creative writing courses she took at Cornell.
This is a decade old, but FWIW:
http://news.cornell.edu/stories/2007/08/creative-writing-program-named-among-top-10-nation
https://english.cornell.edu/creative-writing-program
If he is admitted, he might check whether any of the offerings of its College of Architecture, Art and Planning might be of interest to him (or if he would be able to take them, not being a BFA student).
Union used to offer engineering majors itself, not as a 3-2, has that changed ??
FWIW there is a parent who posts on the CC Cornell subforum who has a child each at Cornell and RPI, he feels that RPI is “warmer”. The students are fine with each other (in my experience), but Cornell’s engineering program is academically demanding. (But well respected). But then, no engineering program of high repute is going to be easy.
Regarding Creative writing generally you might seach for prior threads, eg:
http://talk.collegeconfidential.com/college-search-selection/2007204-colleges-for-writing-p1.html
If he is just pursuing it for enjoyment maybe he doesn’t need a great department per se, just a few good courses (check registrar’s courses given) and profs.
If RPI doesn’t offer creative writing itself, it might have some sort of course-sharing arrangement with other area colleges that do have it. Actually I think that’s the case, you might want to check.