Step son doing weed

Hi I'm stuggleing my step son why lives with us full time smokes weed and will not stop I've tried everything he has suffers mental health for some time due his mother. He lived with us for a few years then took off after getting told off for weed then after year he moved back in with me dad n our 3 children. Now both of us have tried to tell him the rules n that we expect him to stick to it but he just won't we came to compromise with him where he could do it of weekend only has nothing else was working.
Now my childhood is pretty checked with dad's drug use etc and I hate all drugs I'm at the point it's coming between me n dad we have been together for 12 years but i just don't feel dad is really supporting me in this he said he told him it had to stop or it was going to cost him his relationship with me but I dont know if he really did send it. Dad seems to have a issue with standing his ground with becouse last time he did the son tool off to his mother n wouldn't talk to dad at all.
Son is 17 but how I do get past this feeling how do I deal with it do I just give up his clearly not going to stop I need advice as today I'm stuggleing bad and due covid 19 I can't go out alone n get rid of this feeling. I don't even know myself today so please be nice any advice would really help me. Just a side note I love this kid like his my own but his pushing me away by showing me no repect at all