Since you are the stepmother he might be dealing with adjustment issues if you recently married his father. Try to talk to him about his feelings and take his concerns seriously. Living in a split or divorced household can sometimes be difficult for children. Be firm in enforcing rules but make sure the rules are clear to begin with.
Being a step parent is always hard sometimes step kids are unruly because they don't know what to expect from their new "step mom". Be consistant all the time when they misbehave give them consiquences for their actions. When they are good reward them with praise. Eventually they will come around.
Deal with the child firmly but gently, first asking if anything is wrong, second suggesting that he needs to change his behavior, and finally giving him a time-out.
