Are you in the UK?
As Whatisthisfuckery says, there have been a number of deaths and a much higher number of hospitalisations in the US from people vaping black market THC liquid. If he's 15 he is unlikely to have bought these from somewhere licenced, even if you live somewhere it is legal.
www.vox.com/2019/10/28/20936888/vaping-lung-illness-symptoms-death-cdc-report
There have been no cases in the UK but there is also no legitimate THC market here so again, this is likely to be a black market product if he has obtained it here.
I'm not generally one for going OTT when it comes to teens and experimentation but if this was my child I'd be insisting on immediate cessation and handing over all his kit or I would report him to the police.
Keep a close eye on him for symptoms and get medical help straight away if you spot anything:
Patients in this investigation have reported symptoms such as:
- cough, shortness of breath, or chest pain
- nausea, vomiting, abdominal pain, or diarrhea
- fever, chills, or weight loss
Some patients have reported that their symptoms developed over a few days, while others have reported that their symptoms developed over several weeks.