What SSI? It doesn’t say that on OPs main post. If he replied to other people about it then I didn’t read it.
Also reread what OP says “he had to drop classes last semester to keep up with all the household chores I have put on him”. So yes that does put the blame on the father.
Grace? Bruh, this guy keeps getting Grace left and right. A father who is willing and trying to help him get his degree and life on track and rent free in exchange to do chores and help his sister. You make it sound like this 26 years old is 16 and completely helpless. Keep treating him like a child then he will act like one. He is getting older, I am not saying to “neglect” the son like you make it sound like but the son needs to step up. You mentioned he is getting SSI. So he got that going and living rent free.
Like wtf else do you want for the dad to do?
If you are in a situation like OP then let your kid do whatever they want. I’d do what I think is best. Can’t really criticize OP especially all of the family members are suffering one way or another. It’s case by case.