what a rough transition for all of you! I'm sure that he adores his sibling, but is confused about his "new" position in the family. He's having to share his mama - the most important person in the world to him, and it's hard to do! He's used to your 100% attention, and you simply *can't* give it to him all the time now.
I also think that developmentally he is at a stage where limits are for testing! My dd is just about 39 mos and has been constantly challenging boundaries, her own, mine, the pets etc. This is the time for individuation to begin, and children will start doing things that are their own ideas regardless of what is being told to them. It's their job - as annoying and frustrating and confusing as it is.
I would like to recommend the book "Siblings Without Rivalry" and also the booklet "Raising Children Compassionately" by Marshall Rosenberg
http://www.cnvc.org as tools for helping you navigate this new path.
Keep talking mama - even if I don't have anything new to offer, I can always empathize and offer you support from a distance.

Hugs,
~Amy