I really feel for you, had to make this decision a few years back for the same reasons.
Don't feel bad it is the kindest thing you can do for him. He isn't going to get better. You wouldn't want him to suffer. Let him go gently, he would thank you for it. When I did it I felt relieved for my girl and myself. It was the right thing to do. It's not an easy thing to do, but not as bad as you fear, my girl slipped peacefully away. Got to admit took her to the vets twice to do it, and brought her home again with more tablets, which were not going to do any good for her at all. Big hugs for you and your boy, be strong and do it for him.