Step Families Support Group
Traditionally, a stepfamily is the family one acquires when a parent enters a new marriage, whether the parent was widowed or divorced. This community is focused on many of the relationships and issues that may arise within stepfamilies where you may find answers from some members and you may help other members with their questions.
If he hasn't had a check up in a while, get that first, and definitely tell his pediatrician your concerns. Then call the school to discuss concerns about your son. They often have a wide variety of resources for you to pull on.
Meanwhile, supervision, supervision, supervision. And if you think something is about to happen, intervene before it happens. Distraction and redirection are your friends. Make punishments milder and shorter than you'd think. I'd avoid the treats though. You won't get him out of this by rewarding the behavior. You also won't get improvement just by punishments. It's more the distraction, staying attached to him, and time.