What does he want to do? I think he would be a good candidate at schools like Rensselaer Polytechnic Institute and Worcester Polytec and schools like that. The fact that his school is rigorous and he is taking a heavy course load means that his GPA being lower isn’t equivalent to a less rigorous load at a less intense school. His school record and letter from his guidance counselor should make that clear and colleges should understand that when evaluating him. My son’s GPA wasn’t super high and he got in a highly ranked school. I think GPA alone isn’t the only determining factor, so I’d put in apps to a range of schools and put in some reaches for sure, as he has math talent.
Also, I find in some cases some international students think they are not good at a certain subject because they are not as good as some others in their countries, but compared to American students, they might still be good. This is more so true in math and science but could be the other subjects as well depending on the student and school. Just because your son is much stronger in math doesn’t mean he would be weak in other things in the USA. It is also very common for students in certain STEM majors to be much stronger in math and science.
He can also probably get in various NESCAC and similar schools (which are not necessarily STEM-focused but are great schools). Trinity has engineering as a major and a number of others have 3-2 exchanges with other schools for engineering. Bowdoin, Trinity, Bates, Middlebury, Colgate, and maybe Colby all would like to increase the number of Asian and Asian American students they have so being Asian American can be a hook at those schools. His GPA is in the zone for those and math score very high for many NESCACS.
Good luck!! 
I think he is going to get into lots of great schools if you make a list that has a range.
For a safety, depending on what he wants, Rochester Institute of Technology might be one to consider.