Cocaine perhaps, a confidence/resolve boost to help him deal with difficult situations - he will feel increased anxiety and stress in these situations due to the weed use.
Perhaps read up on the effects of these drugs. Notice any sweating, increased/lack of appetite, funny pupils or whites of eyes, really wide open eyes.
Some youth grow out of drug use and stop, some are shocked out of it by something. For some they will become like 'functioning alcoholics' and function in a limited way. And some will experience life-long decline, which relatively few of those will recover from. I wouldn't let this go once you're sure.