How can I get him to behave?
I know he's experiencing normal emotions about no longer being THE baby and I know it's hard for him to understand that he needs to be patient if I'm nursing baby or whatever.
Usually "I know ..." statements are followed by BUT. ;-) As in BUT he has to recognize that there's nothing he can do about it.
I think we fear that if we acknowledge what they're feeling that we're saying we agree and something should be done about it like getting rid of the baby.
But a reality of life is being faced with grieving over what we've lost and can't have again. And life as an only is like that. That life is gone and can't be recovered. He'll never be the special one again. It's not unlike losing a pet. If we lost a pet we wouldn't want someone to say "It's gone and there's nothing you can do about it so learn to deal with it." We'd want words of comfort and understanding about our loss. And if someone comforted us and understood, we wouldn't think that meant they were going to bring our pet back!
A good book about a pet loss is The Tenth Good Thing About Barney. The little boy's cat dies and his father (I think) helps him think about 10 good things about Barney. It might help you understand what he's going through and what he needs if you read the book and substitute "life as an only" in for Barney. Then you could understand better that comforting isn't telling him you intend to give him his life as an only back. It's only saying you understand.
I've heard that yell about 20 times and I know I sound frustrated and bitchy ... no more patience.
If you're hearing that yell 20 times then imagine what it's like from inside him. Imagine what life would have to be like for you to yell like that 20 times a day. And then accept that that's what life looks and feels and seems like to him. Even if you think his life is good and he shouldn't react like that. Accept that to him it's feeling like that.
As others suggested if you ask if he'd like to play in the tub as though you were asking if he'd like to play in the sandbox-- and expect him to say no thanks especially at the beginning -- then you won't be creating the situation that prompts the yelling.
That's just one thing of course. But there's probably multiple reasons for the yelling over various things. He's upset about the new baby so things that would have been shrugged off before are going to seem more irritating. (Think about when you're hyperstressed and just breaking a glass might send you into tears.) He's probably feeling pushed around so even when you ask him to do something reasonable, it's going to feel 10 times worse.
I just feel so bad when he says "are you talking nice now?" Like I'm some crazy maniac mom.
It seems sensible to translate child words into having adult motivations. Try not to. If you with 20+ years of experience at dealing with the world find your emotions stirred up by the words of a child, think about what it might be like to be a child with only a few years of experience. He isn't saying things to hurt you. He's just saying he has needs and they aren't being met. He doesn't even necessarily mean your words at the moment were harsh. He means in the context of his life that isn't going the way he'd like it to, that your words aren't helping his world feel any better to him.
So don't think he's making a judgment on you. When he's saying things like that, he's saying "My life is broken and I can't fix it." What he needs is more hugs and understanding even if you feel like you've given him as much as he should need. An "I'm sorry life isn't going perfect for you right now," will go pretty far! (As long as it isn't followed by a BUT!)
My son knows me well I guess. He uses the "are you talking nice" statement at the slightest change in tone. It always makes me look at how I'm reacting but most of the time I think I am talking nice and I'm not sure how to answer that question.
It's quite possible he's feeling powerless and he's recognized a button he can push that gives him power over you. You can disconnect that button AND help him feel more powerful.
So, by boundaries I guess I mean he's just trying to see if "come here" really means come here or if "please don't play with that it's dangerous" means that or try again.
Don't say come here! Then he doesn't need to test ;-) And if it's truly dangerous, don't trust that he'll put it down. Would you trust that he'd put down a bottle of poison he was about to drink from?
If you're saying it's dangerous and then leaving it up to him you're actions are saying it isn't as dangerous as your words are implying. He should test it out! And if he tests it out and doesn't get hurt, then he learns that your actions are a more honest representation of reality than your words. You're showing him he can't trust what you say.
I don't mean you should snatch away anything that might be dangerous. What I mean is don't water down danger. You want him to drop the running chain saw! But a sharp knife, you can sit beside him and help him with. He wants to hurt himself even less than you do! But he would appreciate help in becoming competent in the world.
It's hard to talk about these things in general terms. Specific examples help a lot so that we can then talk about how to approach problems in a more general way. For example the attitude towards baths can be expanded to everything in his life. Help him get what he wants. If you're treating his needs seriously -- especially the ones that you don't understand or seem ridiculous -- then when you occasionally ask him to accommodate himself for something that's important to you -- like a bath before visiting grandma -- then he'll treat your needs seriously too -- even the ones he doesn't understand.