Could be Kratom. It is bought online, and they put "not for human consumption" on it to please the FDA and DEA. Otherwise, it would be illegal. On the sites, it sells itself by saying, "it's incense for burning" or some crap.
The thing is that he's YOUR child, underage, living in your home??? I'd toss it and send him to rehab before it's too late. That may sound harsh, but you may be saving his life. This is what you actually found.