It is so hard, I really feel for you. 
Our 15.5 year old has been deteriorating with an unknown/unspecified neurological disorder. We initially thought it might be cognitive decline, but he doesn’t fit the symptoms properly and it hasn’t progressed as would normally be expected for CCD. He isn’t as advanced with it as your boy, but we are already soul-searching and having discussions about when would be the right time. Our vet wants us to have a neuro consult and he is fully insured, so we could, but we wouldn’t want to put him through lots of tests or have to hand him over to complete strangers in a carpark. So we are currently logging and videoing episodes for the vet to send to the neuro for advice and to see whether they want to see him.
At the moment our boy is still lucid and interactive, loving walks, food and fuss, still gets excited about things and seems comfortable on his pain meds (although that is monitored closely). He has episodes where he stops, kind of freezes stares, then wobbles and sometimes his head will jerk and eyes twitch. He has also fallen a few times, as he seems to lose coordination in his legs.
He’s had full bloods done and everything came back healthy. Our vet thinks possible partial/focal seizures and suspects vascular/blood flow issues. He is definitely more anxious than he used to be, having always been super laid-back until now and he does sometimes seem to ‘sundown’ but not every night and usually only after he’s got himself stressed about something. We find he’s worse and has more episodes if he is disturbed from sleep or the house is busy and on days where he just sleeps quietly near me he may not have any, although a completely episode free day is rare these days. Stress seems to bring them on as well and he will have an episode if either myself or dh has to go out for any reason, even when left with all three teens, who are all actively involved in his care. He’s sleeping more and more every day as well.
He seems to be completely oblivious to the fact he’s just had an episode and will have one then just walk off and carry on with what he was doing, although often he will walk out of one, straight into another.
We know the day will come when he is more confused and anxious than he is happy and content and the vet thinks his episodes may well progress to full seizures. I read somewhere to make a list of all the things they have loved doing and then see how many of them they are still able or want to enjoy. When the balance tips into existence, rather than enjoying life, that’s when they advise considering pts, sparing them the final days that could mean a big deterioration and medical intervention etc.
We lost our other dog in lockdown1, after a long and horrible illness. We put off having him pts, because we weren’t allowed to go with him due to Covid and he was a highly anxious rescue who was terrified of the vet. In the end he took a sudden turn for the worse and died in my arms at home. We were with him, but it was awful and we both regret not letting him go sooner. Given that time again, we would have let him go at least a month earlier. As a result, I want to make sure we get it right for our old boy.
To be honest, if I was told three weeks, after our experience last summer, I think I would give him one really perfect day, doing everything he loves, say our goodbyes and then let him go peacefully.
It is such a personal decision, but you have given him a fantastic, long life and he knows how loved he is. In the end it is the final thing we can do for them, knowing we are putting their needs first.
I am so sorry you are going through this.