Hi S.,
I am no psychiatrist, but I have an idea as to what is going on. Something similar happened with my little boy.
I noticed your second child is only 4 months old. Although your son is acting well with you and your husband, he might be getting stressed out about the new baby. He obviously loves the baby, but the baby is getting all the attention he used to get. So, he might be taking out his frustrations on people at school.
My son was only 2 when our daughter was born, but something similar happened. We had him well prepared for her arrival, and he absolutely loved (loves) her. However, even though I would spend a special monny-Carson hour at night just with him, he inadvertently got the shaft during the day. Whenever he asked for something, most of the time I would answer "Sure honey, after I am finished (changing, feeding, bathing) Emily." Well, I noticed this (that he was always second gun to her during the day) and changed my ways. Emily was little enough not to notice if I kept her in a diaper for an extra minute or two so that I could get my son a drink, toy, etc. When I made the change, his problems at school stopped and he stopped the tantrums that had started at home.
So, I guess my advice to you is...keep in mind that, although your son loves his little sibling and is behaving well for you and your husband, this whole new baby thing is probably stressing him out. Sit down and talk with him about it (remind him that he is your favorite "big-brother") and make sure that he is not becoming second gun. Make sure he gets some of the mommy-daddy spotlight, too.
I hope this helps,
T.