I've been with my partner for 6 years, we have an 8 month old DD together and he has an almost 18 year old stepson. He's been in his life since he was around 2, he doesn't see his biological dad and his mum passed away when he was 7. He started living with his auntie but continued seeing DP regularly although when we got together and his auntie found out about the relationship, she said stepson didn't want to see him anymore, DP was understandably upset as he saw him as a son but there wasn't anything he could do.
About 2 months ago stepson sent him a friend request on facebook and they began messaging and meeting up, DP found out that stepson had never said he didn't want to see him and that the auntie lied to the both of them. He told DP he hated living with his auntie and he felt she didn't actually care for him and constantly put her children first and treated him differently to them and he was made to feel he was an inconvenience.
About 3 weeks ago he had an argument with his auntie as one of her children (12) had taken his mums necklace and broken it which led to him hitting him which he admits was wrong and has apologised for but this was apparently an ongoing thing where the child would constantly take it to wind him up and unfortunately, on that occasion it got broken. I agreed with DP he could stay with us for a few nights until it was sorted but when his auntie said he could go back he didn't want to as he felt calmer here.
He's been here since and there weren't many issues to begin with but we've since found out he smokes weed multiple times a week and when he comes back here he's been argumentative with DP and has hit him on a few occasions.
With me he's fine and has been helping me with cooking and he's great with DD. We've talked to him about the weed and he's refusing to give it up saying it helps him. We've suggested him go to the GP but he's refused and said his way works.
I feel sad for him but also concerned about him being around DD if he's violent and refusing to give up the weed. WIBU to tell DP if he doesn't give it up he'll have to leave?
I think him getting professional help would be the best thing to do as it's clear his mums death still affects him a lot which is understandable and according to him he wasn't allowed to talk about her when he was living with his auntie which can't have helped how he feels