I'm sorry you feel like you did the wrong thing. Like the others have said so far, it really does sound like he was actually ready. That's the hard thing about pets - sometimes they're ready to go long before you're ready for them to go.

And it completely sucks to have to put them to sleep, but that's part of being a pet owner. It's the worst way to have to take care of them, but it's the way to keep them from suffering.

You saved him all those years ago and got to enjoy his amazingly long life. Think about the years you had with him and all the good memories instead of the last few minutes. Be thankful that your son got to have him in his life for a while. Take joy in the fact that he's no longer hurting.