My brother has Aspergers Syndrome and he went to UIC dental school for some major dental work. He went there because they were the only ones that would take state aid for dental care (at least at the time). They were great. Now, it wasn't totally free - the state aid only covered a percent of many of the things he needed done - but they paid a majority of it. It is a difficult proccess to get started, but once you have your initial appointment it goes pretty smoothly. He still talks about how he liked his dentist there. She was a student, but had a teacher there with her helping and monitoring.
I am pretty sure that Loyola has a dental clinic too...maybe check them out since you live in Berwyn that would be right in your backyard. (addition - I check Loyola out for your - here is the dental website - http://www.loyolamedicine.org/Medical_Services/Services_A... )
Good Luck
P.S. - you may want to contact your insurance regarding continuing his coverage based on the fact that he is a disabled adult living in your home. I know that my M. was still able to carry my brother, regardless of age, as long as he lived with her.