Originally Posted by Morph Bark
It might be a sign that he's become more stressed recently and started smoking again to cope with that. That may also explain why he gets angry when you call him out on it, because he may feel that because you don't want him to smoke pot, indirectly you'd be preventing him from unstressing. Perhaps you should ask him if there is something bothering him?
He did start going to a new school (in bloody Spain - across an ocean) but it seems like he's just doing it for fun. Maybe I should ask if everything is alright.