step son and new baby

Tee-Arna
My boyfriend and his ex have a week on/week off schedule with their little boy (3 in feb). She has a newborn baby and has mentioned that C has started being quite naughty, and is back to wetting himself all the time even though he's toilet trained. I know this sort of thing is quite common but I'm just wondering if anyone has any tips or advice for stopping the accidents and naughty behaviour? Thanks