R.H.
Well i kind of agree with the other lady. He is looking for attantion becuase he lost what he had when the new baby came. He's wanting it back. BUT i don't agree with if you are taking care of the baby and he begs for attention you drop everything and give it to him. What i suggest is you include him in the days events with the baby. Ask him to get the diaper or powder or wipes. Take the diaper to the pail. Things like that. Give him time during the day. Set with him each evening and talk to read books. If he his getting enough he will stop pestering the baby. The reason he's not trying to hurt the baby but it trying to make him cry is becuase it's fun to see what he does can do that... So what i would do when that sort of thing happens i would take him aside and make him sit down on time out. If he throws himself round or won't stay down than bring his carseat in and put him in it, or a high chair. But you need to make it clear that doing what he's doing is not going to give him any attention. ( since children his age don't look for good attention anykind will do) So the least amount of reaction the better.
as Maryam said sometimes it just redirecting him when he's acting up it's always good to try simple things first if you can. Good luck