This is a very potent combination of medications that effect the brain, and alchohol and marijuanna use can lead to unpredictable and dangerous effects. I would agree that this cocktail should be reevaluated by an experienced psychopharmacologist, and your son should know that adding alcohol and marijuana puts him at great risk.
perhaps u can tel him that zoloft is a + drug and when he uses drugs or alchol which is a - it comes out to O help one cancells out the other. if he wants to get better stay of drugs and alchol which are depressants. it may make him feel great for the night but will not improve his future, good luck