Carrie Hagen, Nonfiction Writer and Researcher, Teacher
I can see how this is a tough decision. I suspect that you don't want to remove him because he could do worse in a new setting, but you don't want to hold him back if he might flourish in a "conventional" environment. As a HS teacher who has had many mainstreamed Special Education students, I think that your best resource might be other parents who have made that jump.
Consider contacting a guidance counselor or a special education teacher (perhaps the department head or a head teacher) at the potential school. Ask him/her if he/she could put you in touch with any of the parents in that school's program so that you can better consider this option. If that person doesn't get back to you, ask another -- school websites now should have the contact information for most departments and teachers.