Bless you! I have 3 1/2 year old twins and I can't IMAGINE having an infant, too.
I used the highchair. It worked for me. If he hates that, then tell him when you put him in timeout that if he runs, you will put him in the highchair for his timeout.
Put him in a place where you can see him and put up gates so that when he hops up, he can't get far before you reach him. Don't say a word and put him back.
He is definitely doing this for attention, as I'm sure you know so make sure you are giving him plenty of positive attention. Catch him being good, play with him, have him help you with the baby, etc. If he only gets your attention when he's a turkey, then he is going to act up a lot!
Also, I put their toys in timeout and it WORKS!! If I hear them screaming over a toy, I warn them that it is about to go in timeout and if they continue, it gets put up. If it happens again, I put it in timeout for the rest of the day. They HATE this and most of the time they take turns and share their toys.
Good luck!