Struggling With Step-Family Relationship

I am a new stepdad and my stepson is giving us a little trouble with different things. He is very sneaky—with food, getting on his video games past bedtime and waking up in the middle of the night thinking he won't get caught, stealing food in the middle of the night and being very gluttonous. And when we run to the grocery store or run an errand, I can go on the recent history of the computer and he goes to all different types of porn sites. It's really bizarre to me, because I do not remember doing this when I was as young as him. And I can't blame it on him getting the idea from a friend because he has none—not one! I think that is really weird because even nerds have friends, ya know? I don't get it.

I am having a hard time relating with him and need some strangers' advice out there to see what you guys think. Please give me some suggestions.