Yesterday we rescued my recently deceased grandpas cat after we learned his late wife was violent with him

Hes morbidly obese and old to change his diet. We need to pjt him on a diet, but how do we do that?

Also, he struggles with touch quite a bit. I seem to be the only one he comes to for snuggles, but anything close to his belly or the back end of his body and he snaps at us, whcih is bad since he cant reach there to bathe himself from his weight. We are all allergic and need to brush him to help clean him up, but it isnt really possible from his strong dislike of it. Does anyone has tips on how to help him get more comfortable ?

Also, is it possible to teaxh such an old cat tricks or reaction? Will he ever get used to us ?