Mixing any substances is dangerous.
Cannabis has a direct affect on memory and emotion so undoubtedly it will affect a person’s mental health.
Is your son getting diazepam illicitly? The reason I say this is because there are alot of dealers selling a version of this drug.
“Criminal gangs are selling fake Valium online at a fraction of the cost of genuine tablets, leading people to take potentially lethal doses of illicitly-produced drugs, according to a leading addiction treatment service”.
Tricky if he doesn’t want to stop, however if he agrees to chat with a drugs worker things might change.
Best of luck