It can be really helpful to understand all behaviors as a mean to get some legitimate need met. Kids are not necessarily good at choosing good behaviors, but their need is still real and will not go away if you punish a poor choice of behavior.
Middle children often feel "lost" in the crowd, and particularly with a young baby and a "model" big brother, your son is trying to meet his emotional needs in the only way he feels he has. So it's your job, Mom, to give him better options.
Please try the approach outlined in the wise book How to Talk So Kids Will Listen, and Listen So Kids Will Talk. The authors have years of experience teaching parents how to help their children identify and communicate their feelings and needs, and participate in finding their own solutions.
The book is a pleasure to read, very easy to understand, and it coaches you along a little at a time. It's filled with cartoon examples of what not to say, and what works better. You can probably find it at the library, but you will want to invest in your own copy, so you can make notes and dog-ear pages.
But one of the most essential ideas it presents is that children have a right to their feelings. If you listen thoughtfully and let them simply know you hear and empathize, they can suddenly begin to find resolution, and ways to move forward emotionally. We use this approach when my 4.5yo grandson is has difficult days, and it is remarkable how eager he is to be happy and cooperative once he's been heard.
Better yet, he gets to help find his own solutions to many of his own problems. This will become more and more important as he matures.