It's not a discipline issue, as if he were hitting somebody or being sassy. It's more an emotional thing that you need to meet in a different way.
You want to include him when you take care of the new baby, even though it may take more time that way. Let him do things a big brother can do, like hand you the clean diaper or a burp cloth. When Baby is eating, read books to Big Brother. Let him be with you. Talk to him a lot. Listen to him.
Mention something like, "Susie needs a diaper because she doesn't know how to use the potty. She won't learn for a long, long time. I'm glad you're big enough to know when you need to use the potty! Sometimes you're forgetting now. Would it be better to use a diaper on you again so you won't make a mistake?" He'll probably say no, but if he doesn't, he surely won't like the feel of a diaper after being in big-boy pants!
He may be wondering - although he can't articulate it or even think about it clearly - where his place in the family is now, and he's trying out a number of things to find the answer. Family life has turned upside down for him, and negative attention is better than no attention at all!
So give him positive attention. Stay cool about this, however irritating it is, and he'll decide eventually that he has better things to do.