As a rule, when you don't feel good/are sick/or in sever pain, you don't want to be bothered by others. I think, he is accepting the petting as love and comforting, but he's just not up to doing the 'normal things' from what you are saying.
You are the one that has to decide when enough is enough.
As long as one is willing to eat and drink, I stick with them. This is
willing to eat, not forced to eat.
As a general rule, if the pain is bad enough that it is obvious to you, then he would probably be bad enough that he would not want to eat.
Not eating or drinking would only add to his pain and suffering. When they quit eating and drinking, they have given up.
This is a decision I do not envy you ... I have had to make it myself ... go with what your head tells you, because if you go with your heart you will not know when it is. It's not being selfish, it's just wanting to cling to something/someone you love and not wanting the hurt of missing him to start, or to know the permanence of death. But, watching him in pain is hurting you more than you realize. If you let him go, he will be playing in chinnie heaven with my Mocha and many others that have gone before with no more pain or suffering, just playing freely with no restrictions or problems.
Jo Ann