When to put my dog to sleep?

My dog is 18 years old. He has been with me since he was a puppy and I was 6 years old. the ast couple of years he has not been doing well. He has trouble walking u and down stairs. He uses the bathroom in the house all the time and he never did when he was younger, he always knew to go outside. He has bad eyes and athritis. Some days he has so much trouble getting up I will pick him up to help him to his feet. Then about a month ago, He was in the front yard and someone shot him.I took him to the vet and they said he was doing exceptionally well for his age and that he would be okay. The gunshot dint seem to make him much worse but I know it might add to his problems. I know it sounds like he is suffering and I should think it obvious to put him down but I am hesitant because as bad as he is o some days where he can barely move.... every one i a while he will have a good day where he can wag and jump around ad acts happy.... Its on those days that I cant imagine putting him down.Then I see him on the bad days and how much pain hes in and i cant stand it. Please. any advice would be greatly appreciated.