Congrats on your new little one!
I believe sibling rivalry starts earlier than birth! When I was pregnant with my second and took maternity leave my daughter, who was always happy at day-care, suddendly hated it and didn't want to go. The fact that mommy is home with another child (even unborn) is hard for our little ones to understand. New babies cause a lot of anxiety for our children because they realize it is suddently not all about them. Once your son sees the routine is not too disrupted and thre is enough love to go around for everyone I'm sure things will settle back down.
It is good you've spoken with your son's teacher. She will be much more understanding and encouraging if she knows your son is going through a rough patch. A little extra attention at school can make a world of difference!
It is hard to see our little ones sad and scared, expecially when pregnancy hormones are spiraling out of control. Hang in there ... it does get better.