I smoked cigarettes from a young age. Bitterly regret it now. If vaping has the same hook as cigarettes, I can empathise with your son that quitting will be hard. I think the situational addiction might be the hardest-the first one of the day/after meals are the hardest to give up. Its part of your routine.
My advice would not be to buy him a vape, he already has one. I understand where you are coming from, but cutting down as a form of quitting almost never works for smoking (I am sure there will be a pile on now, but thats what the evidence says).
If he is serious about quitting, I would try and get him to modify his behaviour first to break the cycle. Ask him to think about when he is using his vape and abstain from using it in one of those situations. You could also see if you could get a non nicotine liquid (I remember when I was trying to quit about 10 years ago and an acquaintance had a shisha type vape, no nicotine in it and it was flavoured. I would have happily swapped to that, it gave that "hit" at the back of the throat).
You could go to NHS stop smoking services, the problem you have is the only products that can be offered (if any, I am not sure if they will prescribe to a 15 yr old) are NRT, which is what vaping is.
You could drive yourself crazy trying to find vapes OP. If he is going to do it, he will find a way. He has to want to stop himself.