Hi yes this is a tricky situation, not only do you worry about the drug taking but also the things this can lead to, ie who he is associating with and what other drugs are being used by his friends.
You could check to see if he has another phone, or try and look through his messages as this will tell you a lot about what’s going on in his life.
I don’t want to scare you but I know from personal experience with my own child how things can escalate