My uncle wants me to become responsible for his autistic adult son if something happens to him. What should I know before agreeing?

My uncle recently asked me if I would be willing to take over responsibility for his adult son if something were to happen to him. His son is autistic and currently lives in specialized/supportive housing, so he would not be coming to live with me. From what I understand, my role would mainly involve making legal, medical, financial, and other important decisions on his behalf and making sure he continues receiving the care and services he needs.
I care about my cousin and want to make sure he is always taken care of, but this is a pretty major responsibility and I don’t want to agree to something without fully understanding what I’m taking on.
I’m also not sure what this arrangement is legally called. Would I potentially be taking over as his guardian, conservator, or something else? If my uncle currently has guardianship, can he actually designate me to take over, or would a court have to approve it?

For anyone who has been in a similar situation, I’d really appreciate some advice:

What responsibilities would I actually have?

How much involvement is typically required when the person already lives in supportive housing?

Would I be responsible for managing his Social Security/benefits, medical care, finances, housing, etc.?

Could I ever become personally financially responsible for his expenses?

What happens if his needs increase as he gets older?
What legal documents or plans should my uncle have in place now?

Should there be a special needs trust or money set aside for his future care?

Should I speak with an attorney independently before signing or agreeing to anything?

I want to approach this from a place of caring for my cousin and making sure he has someone looking out for him, but I also have my own family and responsibilities and want to understand exactly what this could mean long-term. Any advice from guardians, family members of disabled adults, attorneys, social workers, or people who have dealt with a similar situation would be greatly appreciated.