Having observed him in a school setting and in the university, and given his traumatic experiences in schools, he seems to be more at ease in the university setting, but happier to play with his peers.
Ahhhh-- see, I think that is a VERY telling statement.
I don't know that I would really try to keep him enrolled in a high school setting at all in light of the problems you've already experienced with below-level work in that environment (that is, a secondary one). It doesn't sound as though he is all that open to the high school social scene, and under the circumstances, I'm not so sure that I'd push it with him. It might be easier if he can parse the different portions of his life with completely separate peer groups. KWIM? This is what my daughter has historically done, preferring to socialize with kids around her own age until quite recently, but commune intellectually with adults or older MG/HG kids. She was pretty intimidated (esp. socially) by the kids 3-4 yrs older than her up until this past year or so. Now, not so much, because they are more INTERESTING to her than her agemates, and they seem to accept her just fine, in spite of her insecurities.
It sounds as though your DS has had reasonably good experiences in the college setting, and not so much in the secondary setting of high school/middle school.
I might just call him "homeschooled" and work college admission from there, honestly.
Have you explored on-line offerings that would be more appropriate for him during the next year or two? CTY and EPGY should have some things that he would really enjoy.
