At least he isn’t smoking it in the house, my daughter does but constantly denies that she is !
You could always come down really hard on him and say if you see anymore in the house you will call the police and he’ll get charged with possession and intent to supply. This may deter him .
If he’s selling the weed then surely he’s actually making money so shouldn’t need to borrow from you.
My teen thinks she knows best and all you can do is be there for him, offer advice and hope he makes the right decisions.
Is he at school or college?
I feel your worry as we’ve been dealing with issues around drugs for some time now!
On a positive note at least he isn’t taking anything else!