my boys new emotions

I have a 4 year old son, who name is Liam. 
We are expecting our second child, who a girl. And we are all very excited. 
Including Liam. Here recently, he's been on top of doing everybthing himself. And really expressing his independence. But with this, has come along a lot of different new bad habits and some very clear out spokeness. 
It's becoming frustrating, because I'm not for sure if it's his way of showing he's ready to be a big brother. Or if it's his way of acting out. 
I don't really deal with rude children, and some over aggressive behavior out of kids very well. So I find myself placing him in time out in his room a lot.  
I hate feeling like I have to be a bad guy all the time. 
I'm not for sure how to deal with this, considering we've never had this issue. 
Any suggestions, for dealing with a only ccild who's not going to be one for long?