Step Son's Weird Behavior

Me and my husband have 2 kids, 7 and 4 and he has a 12 year old from a previous relationship. His behavior is a bit odd to me. He tends to sneak food instead of just asking. I find wrappers and stuff hidden under his bed. He could simply just ask. He also gets overly upset when he gets in trouble. He accidentally broke a class bowl because he was goofing around and when I corrected him he just seemed terrified of me. He peed his pants. I have never seen something like that happen or someone being that scared just being corrected. He was laying in his bed and I went to ask him if he had taken a shower before bed and he starts to tense up really bad when I tapped his shoulder. He didn't answer he just stayed really tense. It's not like that all the time. Just when he's in bed and I tap him.m, or his dad taps him for something. Is this behavior bizarre, or am I reading too much into it?

Sorry, should have mentioned custody arrangements. He sees us weekends, certain holidays, and half the summer.