tbrock, in my other life, I was a superintendent of schools for 20+ years. If you had come to me and asked me what I think about the idea, I would have encouraged you to do it. The opportunity for your son to learn another language, live in a different cultural environment, etc., will be of value to him for the rest of his life.
If your son is serious about his college studies and applies himself, then he will do well in college. The fact that he spent 2 or 3 years living in the DR will make no difference in that respect.
My only question is whether your son is happy about the change. My experience is that once students get into high school that they are opposed to leaving their friends.
I hope your son is excited about making the move as he will have opportunities denied most students in the US.