It is a hard decision, but pay attention, the pups have a tendency to communicate to us when they've had enough. I say talk to your vet, see what could possibly be done, if anything, and then make your decision. It's hard, but sometimes it's be best thing for us to do for them, it sounds like he has had 14 wonderful years of life with a family who loves him
You are in my thoughts, and I hope you find comfort in whatever decision you make.