It could be jealousy, or it could be that he's starting ( early ) to become independent.
Perhaps he just wants/needs more daddy time. Little boys learn from older males. They get to a point where they need to have a man around to teach them how to be a man. At this point they start to withdraw from their mothers.
There are many possibilities.
When my third oldest turned 5 he started that too. He was the most loving little boy in the universe. He would help me cook, clean, etc.
Then one day he woke up and he stopped helping...he started acting strangely...he stopped listening to me, and the only one that could control him was daddy.
It took over a year of psychiatric sessions to figure out that it was severe separation anxiety. He blamed me for having to be away from me...he thought that I was sending him away because I didn't want him anymore.
It's amazing how little brains work.
His outbursts, the not listening, etc was to punish me.
Did all of this start around the same time as going off to school? If so, it could be that...it may have nothing at all to do with the baby.
Perhaps you could ask his teacher...maybe one of the kids in his class has had a bad experience with a new sibling and they have told your son that you won't want him anymore when the new baby comes...I've seen that happen too many times.
I'm not saying that any of this applies to your son...only a doctor/psychiatrist could truly diagnose your son's issues.
They can help you determine what has set him off.
If this continues, I would take him for professional help...hopefully before the baby comes.
Wishing you all the best.
Thank you so much for your advice..my sons outburst don't only happen before school they are all the time when its just me and him but when my husband or anyone else is around he is just fine...the other day I spent a lot of alone time with my son I was baking cooking and brownies and had him help me the whole time which he was glad to do but as soon as we were finished and it was time to start cleaning up for dinner he wanted no part in listening to me until my husband got home...I know he loves spending time with my husband but I have to figure something out to get him to listen to me because my husband just took a truck driving job and will be gone most of the week so it will be only me and my son at home...I really just hope this is a phase