Done correctly, he really won't be in much pain, and he'll have pain medication which will allow him not to hurt. Whether or not to put him through it is a decision you have to make. I removed five tumors from one of my rats, and I was always happy that I did to have that extra time with her.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry