Why not send him to a preschool so he'll pick up English? We know families that spoke something other than English in the home and let the "outside world" take care of the English skills.
I recall one kid in my eldest kid's daycare that spoke Japanese with his parents. (The father was Japanese, mother was American but was fluent.) The first two years of his life he heard very little English. They sent him to a daycare at two, and within six months, his English vocabulary was larger than most of the kids who spoke English only.
He'll need to learn English if you plan to continue living in the US. I've seen HG+ kids like your son learn to speak up to four languages at once - so long as each language has a place or person with which the kid speaks the language. And preschool would give him the opportunity to interact with some other kids. It could be just three or four hours at a time, doesn't need to be full day.
Good luck with the math. It is tough because one teacher can't really customize math for each student in a class of 25. However, I have not seen the other kids counting dots while only one kid understands much more math. Maybe it is the area, but most kids are way beyond counting dots in K (in the schools near here). Opportunities for math are much greater as you get to middle and high school. It is tough to wait but it does get better. My eldest knows multiple kids who qualified for USAMO, including one who qualified before high school. Yes, I'm sure school was boring for these kids, but they are now in great colleges where there are others like them. If you have colleges near you (or good high schools), maybe in a few years you could get a high school or college student to tutor him in math. I've seen others on this board do that.
