Behavioral issues with your stepchild

Hi Mommas! Looking for advice, maybe a little venting and possibly some words of encouragement. For context my partner has 50/50 custody of his 3 year old son and we have an 11 month old son together. In bio mom’s house there’s an older sister and baby brother (mom had a baby right after I had mine). My three year old step son has horrible behavioral issues and lacks discipline in so many different ways. It’s a constant battle to get him to listen to me for the smallest or simplest things and an even bigger issue every time he comes back to us because he is extremely rough with my 11 month old son (where I sometimes feel I have to keep my son separated from my step son for safety concerns), Bio Mom claims he doesn’t act like that over there. I’m at a loss, dad’s at a loss. I feel so defeated by a three year old. It’s almost like he runs my house as soon as he gets here until the moment he leaves…