It sounds to me like what your son and DIL need from you is a huge helping of space.
So a big period of time when you don’t contact them and they’re allowed to just be themselves and not think about you. I’m talking multiple months not days or weeks.
Use the time to build your own life - friends, hobbies, whatever, that doesn’t centre around your children.
If you then reach out, make sure you do so without expectation of becoming a central or enmeshed part of their lives. Carry on holding the boundaries and not contacting them frequently. Realise they are a separate household. In time you may be able to build a healthy, albeit never close, relationship with them.
I don’t know exactly what happened as you’re not able to say, but from what your son is saying to you, it sounds like this is what they need. If your son was forced by circumstances to become your carer when he wasn’t yet an adult, it’s understandable that he feels resentment. It may not have been your fault but you did become something that tied him down and made him less free to live his own life.
The way you speak about “providing him with a roof” etc in exchange for care suggests you don’t really get how imprisoning this must have felt for a young person.
If he even feels like you are trying to pull him back into that - even if you’re not - or as though you want him to go back to being the person you lean on, he is going to react strongly. He has found freedom and doesn’t want to be pulled back in.
I don’t from your posts why DIL has chosen to phrase this as “not safe” - it’s probably not helpful to dwell on these words (or any individual wording) but perhaps she means not emotionally safe.