It's a terrible situation. Overall track down his good days and his bad days. When the calendar is more bad than good days... start thinking about it.
Another thing that helps deciding is, thinking on the top 5 things that he likes to do. Like, barking at the mailman, sleep belly up, etc. When he stops doing 3 of them, then think about it.
I personally kept my old dog too long, she was completely disabled, and I kept her on painkillers too long. Just to be able to hung her, smell her, pet her every day, it was just selfish of me.
Everyone suffered with the delay, the other dogs could had better vet care but hadn't, due to the expenses, and the other dog that I adopted later, spent unnecessary months at the shelter. My human partner, is now done with more dogs too, because the process is a bit tiring.
Also, make sure that you do everything right by him, or you might end up like me, needing another elder dog right away, just to re-try everything to make things well "this time".