This must be so hard for you. Be firm but gentle at the same time. Forget about punishments or time outs. Logical consequences will work. If he refuses to pick up a toy, no problem. tell him I can see you no longer want that toy and remove it. No discussion or argument follows. If his angry response bothers you tell him you cannot tolerate that noise and go to your room. Be direct. Do not ask him to do something as he will say no because he can. Use words like, it is time to clean teeth now and then we will have story. If he refuses to clean his teeth a gentle reminder that there is no story till teeth are cleaned should work. If he stalls too long, you respond with what a shame you took so long to clean your teeth now we can only have a very short story as it is sleep time.You need to eat breakfast and then we will go out. If he does not eat breakfast, no problem. What a shame you did not eat your breakfast now we will have to stay in.
If he persistently refuses to comply with requests. Respond with take as long as you like but nothing else will happen till he does it. Use a clock to let him know how long it takes. When the big hand is on 6 we need to eat dinner. Any lego not put away will not be there in the morning. If he refuses to go to bed on time remind him that he will have to go earlier the next night to make up the lost sleep.
At the same time try and put some fun into the every day. I wonder if you can get dressed first. You are so grumpy I think you need a tickle. Do not let his defiance and anger spoil your day. When he yells at you remind him you will only listen to his jnside voice, When his behaviour is very trying send him to bed for a nap because you can see he is really tired or he would not behave that way.
I do not believe in time outs unless I the parent take one to give me some time to calm down. Sometimes I just walk away from a confrontation and make myself a nice hot drink in another room.
Work on one bad behaviour at a time. If his aggression is a problem make a poster with the rule we do not hurt anyone in this house. If he goes to hit you, grab his hand and hold it firmly but gently. Ask him Do I hit you? do not hit me and walk away. All scoldings need to be very short and then you move on. You might need to talk to him when he is calm about why hitting etc is not acceptable and what he can do instead. Read him books from the library that address this problem. eg Hands are not for hitting.
With a new baby in the house you will not be as available as before. Ask friends and family to help out by taking him for a playdate or outing. Make sure his usual routine is not too disrupted and he is eating regularly. Have special activities and toys that can only be played with when you are nursing. It can be a new dvd, small construction set or puzzle or craft activity. Stock up at the dollar store.
Affirm him every time he manages to wait a bit while you attend to the baby. You waited so well and reward him in some small way. You can have an extra story at bedtime. Tell him the baby is so lucky to have him as a big brother. He should also get some big boy privileges the baby cannot have.
You can also read stories to him while you are nursing. Involve him in the care of the baby. He can fetch diapers,bring you a drink of water, answer the phone and help you bath the baby. Just take it day by day and do not worry about the future. He will mature.