@slingingshot15 There’s a lot of good suggestions here, but from one step to another, you need to talk to your wife first. Of course there’s a good chance this has to do with jealousy or feeling uncertain about his place in the family, but it can’t just be you as the step addressing it. That puts you in a awkward place if then she or bio dad don’t like how you’re addressing it. But I do think it should be addressed. He might benefit from therapy, talking with another trusted adult (like a grandparent or aunt/uncle), and definitely creating and maintaining hard boundaries. For example, if he starts kicking she can say, “I am not going to let you hurt me. You can be angry, but you cannot hurt people because you are angry” and then either remove him or yourself. After he calms down, discuss with him the boundary that was crossed and what the consequences is (ex. no TV time that day) then hold that boundary, even if he gets angry again. If he physically lashes out again, repeat the cycle and extend the consequences (now no TV for two days). Kiddo might really push it. If it seems like he’s just in an out of control cycle, he needs a long break and honestly some time alone in his room or a quiet corner to break that might be necessary. We purposely don’t keep a ton of toys in my stepson’s room for that purpose, also to discourage him from playing with toys when he should be in bed.
It’s hard, especially when it’s a stepchild. My SS is 5 is having a really hard time adjusting to my one year old. He never lashed out when I was pregnant, but definitely lashes out at the baby now. Sometimes I get a very protective primal urge to protect my baby from him and I have to really use my grown up coping skills (lol) to remember that he’s a kid and his brain isn’t fully formed.
Lastly, keep talking to him and practicing what to do when he’s upset. If he’s like my stepson, it will literally take years for him to start using those skills. But it’s worth it to instill them now.
Good luck with your growing family, and congrats!