It sounds like his quality of life hasn't diminished much. I just can't see doing anything drastic until you can see and feel that he's in a lot of pain or totally unable to move.
I know it's a hard thing. I had rats growing up. One of our rats, Scampers, developed a tumor on her left side. We had it removed and she lived for almost one more year until she developed another and died. I was only seven and I still remember how it broke my heart.
I don't think its selfish to want to keep your pet around as long as possible. I think you will know in your heart when his time is up. I hope it's not for awhile
