Our old girl had a retching style cough because she had a heart murmur, but with medication she was fine. In her final few months she became faecally incontinent-her hips were too weak for her to squat, and you could see her muscles involuntarily making her poo when she was lying down :( she didn't notice, and it wasn't her fault, it was just gruelling for us to have to do emergency clean ups in the middle of the night.
Puppy training pads were handy to put under her when she was lying down.
This sounds awful, but she was actually still happy, interested in food, wanted little walks, tug of wars etc.
We avoided the vet because ours are utterly useless, and we were doing intensive care for her, there's nothing the vet could have done anyway.
One day she made it very clear she was in pain and had had enough, her whole face and demeanor changed, and we knew right away that was it. The vet came to the house to give her the dreaded injections after we'd all spent hours cuddling her and telling her how much we love her. She was gone within seconds.
So, take him to the vet if you like, but they can't forcibly kill him against your wishes! You know him better than anyone and if he's still enjoying his life, in an older dog, slower way, keep him for as long as you can. They are like puppies again when they're old-the toileting, neediness, need help, etc. but just cuddle the hell out of him while you still can, life without them is so so horrible.