Defiant and rude

I don’t really know what to do atp, my step son is 5 and is down right just rude and doesn’t want to listen to nothing I say and I take care of him 5 days out of the week by myself while his dad is away at work he sees his mom sometimes once a week it had been about a month the last time he seen her before last weekend he spent a few hrs there and he comes back a monster, I don’t really know what to do or why he’s acting out like this, it causes a lot of tension in our house but that month he didn’t see her he’s been a great kid . I’m really at a loss, his dad says that there has to be a reason but literally nothing has changed except he seen his mom for a few hours the other day. Literally these last 2 days have sucked so bad and I feel like his dad is starting to be mad at me about it. I’m really trying my best and at my wits end w it🥴😭Any advice?

Edit: he hasn’t been over there bc he doesn’t want to go and his dad doesn’t make him go if he doesn’t want to and the problem is is that there is no discipline at his moms and we have rules at our house. Now I want to add no I’m not bitter I have no issues w his mom it’s just very annoying that every time he comes back it’s like hell dealing w him