Yeah. He's definitely got some issues. He's probably angry. His life has clearly been changing and when you're a kid you don't really understand and you can't fix it.
I know I really hated feeling like I had absolutely no control over my life. My life when my biological parents was ROUGH, then when they split it was lovely. My dad was wonderful and kind and we had a lot of fun and always felt comfortable and loved. Then my dad remarried and it was weird having to share his affection. My mom was young and didn't really understand kids, so she tried to reason with us and treat us like adults (which sounded good! Except that we ran all over her, then she would lose it.). Then she got pregnant and I felt really left out. I felt like my dad loved her more than he loved my old mom, so he would love the new kid more than he would love us.
I don't claim to know what your step-son is going through, but having a relatively new step-mom and then also getting a new sibling thrown in the mix could be doing a number on him. He is likely fine with everyone else because they aren't as new as the unit of you and your husband. So he's testing the boundaries and seeing what you guys will respond with. Honestly, he might be hoping that if he's bad enough, you'll leave. I know I did that. I felt like my bio-mom left because she couldn't handle kids (true) and if I pushed hard enough, my mom would leave and it would be back the way I wanted it.
Again, all just useless conjecture basically. Maybe it can give some reference or insight though. I don't know. I'm sure you've considered it, but maybe counseling. Most of the time with kids counseling there is a portion of it where it's the whole family. It might help to have an objective mediator. It might also make the kid feel like he is part of your family (which may help if this is how he is feeling, especially with a baby coming).
Has his behavior changed/intensified since you got pregnant? Just curious.
[Side note: MOST 12 year old boys won't brush their teeth if you aren't watching over them. I was a boy scout camp counselor for 8 years and I would make staff in my area keep tooth brushes in the area and have them brush their teeth each day before morning meeting. Without that, they'd have gone weeks without doing it! They were 15-18!]