Please please talk to teacher as soon as possible to find out what is going on there. I would be very concerned if my child suddenly started to withdraw, it can be a bad sign of something like bullying. Suss it out promptly.

If there is nothing going on, then it could be just an adjustment issue that the teacher should be able to help with. Talk up how great kindergarten is and invite some of the other kids over for playdates to help your son make friends. If things don't improve, I would seriously consider moving him to another kindergarten. Life is too short to be unhappy