You're going to have to let your vet and your heart lead you on this one. I am so sorry to hear this. You and Coach have come such a long way in such a short time. See, you have bonded. If there is a chance that he can come through this the rewards will be immense. If the vet believes that his suffering will be too great, then you have the comfort of knowing that you have made his stay with you one of love and care and that he loved you back enough to pee on you.
There is a reason he has come into your life and there is a reason he will either stay or leave. My thoughts are with you as you make these difficult decisions.