Determining quality of life is always an impossible decision. We've had to make it too many times already. Only you can really determine the minimum quality of life you want for your pup and when it's time to say goodbye because he's at that point. Here's a site that has helped me over the years:
www.pet-loss.net
The breathing does sound like it could be laryngeal paralysis. We have a greyhound diagnosed with it, but wasn't a candidate for surgery. We were able to manage it a bit with meds - valium and doxepin. It gave him a couple extra years. Though, we had a terrifying episode where he nearly choked to death before we were able to get him to the hospital...I promised him he would never have to go through that again. He was clearly terrified. We said goodbye shortly after that....
Good luck with your pup...and hugs.