Your son sounds like a normal little boy with a new sibling. His acting out might be an attempt at getting attention since his new little sibling is taking mom and dad's time away from him. I don't know if putting him in summer school is going to change that, but if he's missing the social atmosphere of school and missing being with kids his age, then the school might be good for him. Learning not to yell for attention will come with time - don't give in to him when he behaves in a manner that is not acceptable to you. Tell him he's hurting your ears and you can't understand unless he talks in a normal voice. (when my kids did that, I covered my ears and pretended it hurt). Does he get much of a physical outlet? Does dad get down on his level to play with him? My guess is he's wanting something from Dad. Can Dad find ways to get him to be daddy's helper? Do something special with just him?
As far as learning religious stuff, that's definitely something you'll want to consider. Personally, I think it's great to expose kids to all kinds of religions (tolerance and diversity). Regardless, you will still be the biggest influence on him over his beliefs. Maybe you can visit the school for a day and see what you think - my guess is they are not getting "preaching" at all, but are learning about love, being kind to others, being helpful, having fun with games and activities, etc.
As a mom of two boys I can tell you they may not "mellow down" for a long time! Mine are 14 and 16 and just starting to mellow out a bit. :)