Nicotine or no nicotine, I'm not sure it's actually illegal, and I would assume that the laws vary by state.
If it were my son, I wouldn't exactly want to encourage it, but I'd also recognize that for a very long time, it's been outright illegal for people to smoke under age 18, yet that has never stopped teenagers who wanted to smoke from starting to smoke. That includes me. My parents didn't smoke, but at some point in high school, I picked up the habit anyway, and I had that habit until I started vaping about 3 years ago.
I wouldn't completely trust him when he says "no nicotine" and "I don't use it at school". The test kit someone mentioned above sounds like a good idea.
My GF is a high school teacher, and she's quite familiar with vaping due to hanging out with me. She's well aware that a lot of the kids at her school will vape at school, in the bathrooms, etc. Some of those just vape, and some of those vape at school and smoke cigs when away from school. Vaping is much easier for them to get away with since it doesn't have the strong cigarette smell and such, and it's a way for them to get their nicotine fix when they can't easily smoke.
I wouldn't want to encourage it, but it's much better that he's vaping compared to smoking, and I'm pretty sure that a "I'm putting my foot down, drawing a line, you'll never do that!" approach is unlikely to be effective. So I think really pushing for "No smoking, period" and discouraging nicotine in his vape juice is the approach I'd suggest.