K.H.
Everyting's going to be OK. Truly, it will. There is nothing physically or mentally wrong with your 12 yr old. Not sure if you're a reader, but ther are literally tons of books on this very subject. Try and find one that fits close with your situation. Counciling is always good. For you and/or your boy. It sounds like you both could benefit from talking with someone. It's tough trying to teach coping skills, but I've learned to keep an open line of communication with my oldest boy. Talk to him. Tell him what he's doing wrong and see if he can find his own solution. He'll probably do the usual huff & puff oh that's stupid routine, but stick to your guns. I've found that I needed to celebrate all the little things my guy does do. I have to make sure he knows he's appreciated when he does do something right. Sometimes I forget to do this.
Are there any consequences to his actions? Is he held responsible for them? This can be so hard to do especially when they think they know everything and they tend to have selective hearing, if you know what I mean. 12 yr olds can be mean, but I don't think they know how to express themselves properly.
Have you tried contacting the school guidance councilor? Sometimes they can help too. My oldest son had visits with his twice a week until his grades came up. Sometimes it takes someone else to get them so see their strengths and open them up to becoming an adult.
You're not alone in this, trust me. It's so frustrating, but know that you are a good parent. You and your husband know him best. Find out what he's so angry about and see if you can find a solution together. He needs you both now. Try and see past the fresh mouth and dig deeper. It sounds like you've got a lot going on there with his biological mother, being a step parent, and also having the new bundle of joy. Are you taking time for yourself to recharge? I know, I'm laughing at myself for saying that because I can't seem to find the time myself.
A lot of times it boils down to the kids not knowing how to cope with certain feelings and emotions. Perhaps this is part of his anger. We weren't taught how to deal with our emotions, so we all find our own ways of coping. Have patience, be understanding, but make sure he knows where his boundaries begin and where they end. Make sure he knows what the consequences are when he crosses them, and make sure you follow through with every, single thing you say. If you don't, they'll be empty threats that he'll catch onto and call you on it. I figured that one out the hard way...
Very cunning these young people, yet they're so filled with love. Sometimes you have to dig real deep to find the love, but it's there. Keep in mind that we parents become more and more stupid as they begin to mature. It isn't until they're oh say in their mid to late 20's that they finally realize how much we actually know. It's pretty comical.
This is what I've found with my kids. I hope it helps!