I'm so sorry, it's agonizing to lose a member of the family like this. For what it's worth, I agree with smeddley. He was in pain, and not able to get around. If it hadn't been for you and Jason he would have been gone 17 years ago. If you hadn't done this now it wouldn't make it any easier than if you waited for him to get sick, or to be in so much pain that you wished you had done it sooner.
You have almost two decades of memories to overwrite that last three minutes. I know it won't be easy but eventually you stop remembering the bad stuff and cherish the good memories. :)