Terrorbull, I really feel for you. These decisions are never easy. I think you are making the right decision. If he is in so much pain, it would be cruel to let him continue on like that. As hard as it is to let go, it's the best thing for him and for you because it must be so hard to see him like that. Just remember al the good times and know he will forever be in your heart.
Those we love don't go away, they walk beside us every day.
My thoughts are with you.