Been there,done that nosha........don't feel guilty-just do the best you can for him.
My cat Cleo was diagnosed with oral cancer almost a year ago.....she had been dribbling a bit-which I was not too concerned about. I had had dribbly cats before-it always seemed to be a sign of affection in them. She also became fussy about her food-as she was a realatively young(5yrs)cat,and I had had her just over a year-I thought she was just doing what all cats do-driving me crazy! But by mid-September the dribbling had become copious-she was a long-haired cat,and her fur was full of it.I needed to wait til I was paid-another 10 days or so-before I took her to the vet. They initially diagnosed a dental problem-which she DID have,and she was scheduled for surgery2 days later. I had a call from the vet saying that they had found what could be either a massive infection -or a tumour. I felt as if the floor had been pulled out from under me. When the final diagnosis came 2 weeks later,she was given 6weeks. I spent a good deal of that time beating myself over the head---all the 'what-ifs'..if I had taken her sooner would the prgnosis have been better? doubtful as the cancer was in her upper jaw-near her eye.
Sorry if i am rambling here.....just be there for him-give him all the love and comfort you can. He will tell you when he has had enough. x