if he is a danger to himself, to others, or is overtly angry or violent, then medical intervention is necessary. beyond that proper discipline and perhaps religious counseling if you are into that.
Hi. Well, it sure can be hard ot blend families. Are you already married to his dad? Was he in the picture while you were dating?? I'm just asking because I'm trying to figure out if this is a change in behavior. Was he easier to handle before you were married or lived with his dad and then starting acting out after you all were living together??
Could he be jealous? It is really hard for a young child to understand all the dynamics of shifting households of parents love lives. If he hasn't other siblings elsewhere and this is his first experience, I'd cut him some slack as he adjusts. He may not be used to another child in the house his same age all the time that he must share with, gets equal attention to him (along with 2 other kids), etc. He's young enough that reason will not work and his feelings are undersandable. BIG change for him and kids sometimes have a hard time with that.
I'd have his father spend some one on one time with him away from the other kids. I'd also try to do a bit of that yourself with him. And then he may feel more a 'part' of the house. You may feel this is rewarding his bad behavior but in reality, it is setting up a bond with him and making him feel SECURE in his new situation.
His dad should handle discipline for now so that he begins to see you as safe but you can be behind the scenes working with his dad. If you are taking care of him while his dad is gone, then for the time being treat him like a special guest. Again, may see unfair but he probably FEELS like a guest in your home because it doesn't feel like home yet.
Eventually he'lll settle in and then you can mix some tough love into your love of him. It helps to try to see things/life through his eyes. He's a visitor in a house that his dad now lives in with a bunch of kids.
good luck