I recently saw the Gordon Page, Jr. segment on the old Unsolved Mysteries show. Extremely sad story.
It's very unfortunate that he was mis-diagnosed for so long, that he couldn't hold down a job due to this disability, and that the last time his father saw him (at the group home) it was obvious that GP wanted to come home.
I completely understand why the parents didn't have GP living with them. He couldn't function well, and as such would need constant care that they probably couldn't provide. My understanding is that it is financially less expensive to place someone in a group home (which in some cases is state-run) than to hire 24-hour care for a person at a residence/private home. In any case, it was heart-breaking seeing the parents interviewed (especially the father, who is now deceased); they both were very likeable & were obviously very concerned for their son.
I suspect that GP just wandered out of the group home & ended up homeless somewhere, or possibly ended up in a shelter/another group home. However, it's unfortunately more likely that he's deceased. I'm not surprised he has never been found. I.e., since he himself wouldn't have been able to contact his parents & since he probably didn't have any contact information (for his family) on his person, it seems evident that he probably just slipped through the cracks somewhere. Very tragic.