The advice I received that worked was be consistent in contact, though not pressuring. Invite him places. Ask, Can I do anything for you today? (I forgot the exact phrase, but something like that is what a counselor told me.) Also, sincere compliments and shared interests are good for any relationship.
I would let your son bring up subjects like the illness or hospitalization or social isolation in his own time. He has a doctor and is med compliant. He lives independently. That means he's doing relatively well.
It took about a year and a half to rebuild relationship with my family member after police dragged family member out of our house following contact with social services that I initiated during family member's severe episode of psychosis.
Now, due to recent severe episode of illness, things have gone extremely wrong in family member's life, but we are united as a family and able to try to move forward.
I hope for the best for your family.