The Chemicals you can smell/taste are more than likely within the weed that was sold to this kid.
I have some "friends" in the Police force and as a Property Manager, we had 1 incident of finding this stuff on a premise. The Drug dealers put the chemicals into the weed, to give a bit more of a high than usual to the person taking it, making them want to come back to them only, for more.
The chemicals are dangerous, can make a person sick.
Your Son is putting chemicals into his body. It sounds as if you are ok with weed and you know enough about it, smelt it even and I won't say anything in that regard, after all it comes from the ground not man made.
BUT, the chemicals these big wigs put into it and on-sell it, is man made, dangerous and will cause health problems.
You need to educate him if you are going to accept him taking weed that what is out there is laced and dangerous, as you suspect. Perhaps google it and find some horror stories to deter him full stop.