That’s actually quite common, esp in some paired classes (eg, micro/macro econ; comp/US gov), and is what he will already be doing with Mechanical and E&M.
(sidebar: never underestimate what schooling systems will do to students: when my lot were in secondary school overseas they were required to take 3 languages, and the exams for different languages were on the same day; and one of our kids had final exams in a UK university that covered 3 years of work!)
To me it does look like a serious stretch, even if only in terms of raw time: M + E&M in one year is a real push. Bio is time heavy. Depending on the school, AP Lit can be time heavy (lot of reading/writing papers). I don’t know about the Data Structures / Game Design classes specifically, but the people that I know who play in that sand box like them a lot, and seem to sink a lot of time into them.
Moreover, a lot of people round these parts suggest considering writing college applications nearly a class in itself. It takes a lot of time- and it is not easy time. Getting started on essays in the summer is fine, but your son is (developmentally) morphing right in front of your eyes at this point, and I have yet to see an essay written in July that doesn’t need re-writing in Sept / Oct. Given that for some of the schools he is applying to, it is the essay(s) that really differentiate one high-achiever from another, it’s not an area to shortchange. Moreover, I’m assuming that there are ECs of depth and commitment that will require time.
On the other hand, a STEM school that will both require some of that and will allow that schedule suggests that there should be some good local data sources (older students & their parents). If the acceptances match, the GC should be able to tell you how ambitious that schedule is, and how other students (in general, obviously) have done in handling that sort of course load and in navigating college sweepstakes.
So, the best I can offer is: you know your son, and with any luck his GC knows him and how those classes play out in his school (there is a lot of variation between schools). How does your son sit in his cohort?
Also (what I should have asked first): is he looking forward to this schedule, or is he doing it because he thinks he ‘should’ or even that he ‘has to’?
Also also, in real life, if he gets into the term and is overwhelmed, is there a realistic escape clause (ie, schedule adjustment process)?