Hi,
We dealt with this two months ago, although a rapid two day decline probably made our decision a bit easier. It was a no brainer in the end. The light went out in her eyes, she wasn't interested in anything, even us, and we had to hand feed her as she wouldn't even get up for food. The guilt I felt just in that two days for the pain that she must have been going through was horrible. Holding on too long is my biggest regret; but honestly she had never given such definite signs before, only old age, and I guess I needed to see definite signs to know that I wasn't cutting her life shorter than she wanted.
We held her as she was PTS. She knew we loved her.. It was tough, and still is; I am crying as I type this. But I know we did the right thing FOR HER and it is what she wanted. I couldn't stand seeing her like that. If your boy is in pain, or showing that he has had enough, your greatest love for him is giving him his wish.
Best wishes. It is tough.