OP,
First off approach the teacher to fill her in on what is going on.
Decent teachers always appreciate being informed of background information that informs class dynamics.
Do it.
Next, get your child involved in local sports activities that will build his confidence, help him meet other children, and fill up his week.
You cannot change this, so if you are insisting on keeping him in the school, work around it.
I presume it is not every child so you need to start other playdates and make them the best you can.
Ask him about the other nice boys and bring them over.
Even put yourself out and have them for a late Saturday afternoon/early evening and do a pizza night and a fun movie.
See this as a project and get serious.
But remain bright and breezy with it at the school gate.
You have YEARs and Years ahead of you.
Things change and so do friendships.
She is a controlling headcase and most schools have a couple, so best just actively avoid her.
Parents that will join in the active exclusion with her are best avoided too.
Be glad of the heads up early.
Many people are years in before the headcases expose themselves and the damage is worse.
So get busy making school just ONE part of his life.
Soccer, cubs, tennis, rugby are great sports to get involved with.