I started smoking at 15 so, 45 years before I could switch. Still, I started vaping in June and just last week was my first without cigs. At first it was almost seamless. I'd have 4-5 cigs a day as opposed to almost two packs. Then it was down to 3-4, 2-3 and finally gave up the last.
Overcoming an addiction has a lot more tied into it than chemical dependancy. There's physical habit, a huge sense of loss for me, a fear of being without a pack and a Zippo...many things to get past.
So, give him time and space and support. Check on him, but gently.
It's a huge step he's already taken with your help.