Hi KittyB4me (did you have a cat sitting across the keyboard when you made that screenname?
You sound like you really have a plateful! As does your son!
I'm guessing that your observations about all the stress in your son's life are correct; a lot has changed in his life in a short time. Somehow, that stress all has to come out and at 30 months it's not going to come out in well-considered and thought ful ways. I know it breaks your heart the way he treats you, but I'm pretty sure you're getting the brunt of it b/c you're a SAFE person and he knows he can trust you to be there no matter what. (smart kid, eh?)
I'm going to ramble some observations or ideas and you can see if they'll fit or look right with your situation. One thing I see right off the bat, is the potty training. I'm not sure how that's going with you (you only mention it as a factor) but I can share what happened with my first ds. I got to the point where I felt he ought to be potty training and instigated what I felt were appropriate methods (all very gently, low pressure, high reward, etc). Ds had other ideas. The more I pushed the idea of getting out of diapers the more stubbornly he clung to them and in the end it all turned into a very stressful power struggle. He honestly just was not ready. I had to back off and let it go- when he was about 3 and a half he was ready and it happened very quickly and easily. I wonder if with all the stress and with the competition of a new baby if he is feeling like he wants to hang onto the baby-ness of diapers. You and I see them as a mucky mess, but to him they may represent a measure of security and of his world not changing any further.
At that age, with my first ds, I found when he was flipping out about somethig and hitting or just screaming or whatever migraine-inducing behavior he came up with, it helped a lot (well-not right away, but eventually) to just observe out loud to him what I saw going on. Giving kids words to express themselves is really, really powerful. It may not alleviate what is happening at that moment, but I promise you down the road it will help them SO much to talk their tantrums out. So I'd say things like "wow- you look really frustrated that you're not getting X. You must have really wanted it. You're so mad you're shouting and yelling! You're so mad you want to hit! YOu must have really, really wanted that (etc. etc. ad nauseum) I can really recommend the book "How to talk so your kids will listen and listen so your kids will talk" it's written more toward an older crowd of kids, but I think the skills are just perfect for toddlers and preschoolers too. Many times what would end up happening was that ds would finally break past the wall of his rage and relax tearfully and start talking about what it was he was really feeling. Sometimes it wasnt' even the issue that triggered it all in the first place.
Eating and behavior: yesirree. Not only whether they are eating enough, but sometimes also WHAT they are eating. Sugary foods (including fruit juice) are horrible for our family. HIghly processed or dyed foods are also bad; there are certain brands of baby tylenol I cant give my ds2 because the coloring (i think) gets him wound up past belief. We've made a lot of changes to our diet because of what we observe between diet and behavior, as well as health issues. I try to keep some kind of healthy snack on hand at all times: a box of whole grain crackers in teh car, a couple of hard boiled eggs in the fridge- it can really be a lifesaver.
My last ramble: try to make some time when you have just you and ds without the baby. He may be speech delayed, but I bet he understands EVERYthing he hears. You could try a few conversation starters like "sometimes I imagine you get really upset about having a baby and you wish things could be like they were before" He may not respond, but he may. And even if he doesn't it will give him the message that you DO see and understand what is going on with him and that little bit may help him to relax a little. You don't have to make apologies or try to "fix" it for him- just an openended statement and see where he goes with it.
I lied: one more. Sensory things like playdough or a bowl ful of rice or birdseed and a few little scoops or pourers are somethign my ds1 found VERY soothign when undergoing stress. He would zone out and run his hands through the bowl for a long time and it really seemed to help.
I'm sorry this is so long: I've been there, (as have all other moms!) and hope some little bit of this can help in some way.