I am so sorry PinotNoir. I haven't had to make that decision yet (though it will be coming some time soon), but I have lost two kitties, and it is devastating. Everyone here who's lost a kitty understands how deeply we grieve for them, which is so comforting when we're surrounded by people who think we should be "over it" after a few days.
If it helps, and when you feel up to it, you might want to post in the Rainbow Bridge section of the forum. We would love to see pics and hear a bit more about your boy.
Since you knew he was miserable, you made the right choice in letting him go and not making him suffer. Poor little guy. He's in good company where he is, along with Dizzy, and my Margaux and Olivia.
Hugs and much sympathy.