I can appreciate how you feel FD, when I had my old bitch Gypsey put down some years ago I cried like a baby.
She had no real quality of life left so I did what you will do and it is the right thing, you'll look back and always know you did right by him.
One thing I think you should do if you can, is find a spot in the garden or a place he liked to go, dig a hole and bury him.
It will make you feel like you did this last bit for him, it made me feel better anyway.
Be strong.
Shepp