I agree with Tony. If his life is pretty good generally, then it isn't time to be put to sleep. If his insulin needs to be increased, then so be it.
I hope you have a good vet on his case. If you have any doubts about the vet's work or advice, then seek out the advice of another vet.
Don't give up yet.
The thing with suffering is.....it's time to call it a day when nothing works any more. Like all of us -even dogs have to put up with some discomfort, sometimes. So long as there is hope, and medication can still help, and the dog's life is pretty good -keep trying.
You have done an amazing job with your nine year old and I can fully appreciate how difficult things must be, but clearly if you can get the diabetes under control life could still be good for both of you (at least for a few more years possibly). I would encourage you to keep trying for as long as you can. The injections are more a nuisance than painful (as a diabetic I can actually say that), and if he is otherwise contented, then it must be worth persisting. Tony