Struggling to bond with my step son

My step son is 15 and he started living with us full time 8 months ago and I've been struggling to bond with him the whole time. He was legally removed from his mom's house because she refused to leave her abusive boyfriend and he had been witnessing DV between since he was 9. His mom and him decided to move to a different state and he hasn't heard from her in 6 months. He's an extremely reserved kid and doesn't have a single friend. He's also been dealing with being bullied. He sees a social worker weekly and his therapist comes to his school to see him. We've had some issues with cutting back in May. He also sometimes wets his bed. So he's struggling with many things. Me and my husband have a 5 year old daughter together and I want us all to be a family but I've struggled to bond with him. I've tried to be there for him and show him he can talk to me but nothing seems to be working. Yesterday I was trying to help because my husband tells him if he has a bad night to put his lennings in a corner in his room and my husband will get them to wash them. I was checking on him and he said he was fine. His bedding was in the corner so I said I would wash it for him. He was clearly embarrassed and said his dad would. I said I don't mind and he snatched them out of my hand and said no. We all know he has this problem sometimes. I was just trying to show I don't judge. I've tried bonding with him and I want to be an adult in his life he can count on. I just don't know how to do this