This is really confusing. Do you have ANY family at all where you're at? Is it because you have no one that you use his family as a surrogate family?
As for his mom's request, you should explain to her that you and her son do not have that type of relationship. If she understands that and still wants your help, and you want to do it, you should tell her you're doing it for her and no one else because you care for her. Although, it probably will be awkward for everyone. Especially you.