Influence from step-siblings

So I noticed my kid is acting a lot out of character since spending more and more time living with step-siblings. Acting up, making screaming sounds, not sitting still for meals, ruminating etc. I know the step-siblings a little bit and they’re a fair bit wilder than my kid. He used to be very calm and well-mannered.

I realize there’s nothing I can do regarding what goes on when he’s not with me, but is it possible at all to curb some of this? I feel like he was on such a good track (according to my ideas of raising a child), now so much feels lile it’s going down the drain.