Need help from my fellow Moms!

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Hi ladies! My son is 5 and just started kindergarten this year. We put him in Pre-K last year, but it was only 4 days a week for 2.5hrs a day. Now he is doing full day kindergarten, Mon thru Fri, but he hasn't been adjusting well at all. The problem has not been with his ability to learn (he's a very bright kid), the problem has been behavioral. This is the third call I have gotten this week from the school principal saying he has been very defiant and uncooperative in class. I was going to go down there again, but she advised against it, saying she doesn't want to get him in the bad habit where every time he is disruptive in class he gets rewarded by having mommy come have lunch with him or take him home early. I completely agreed, and decided to have a talk with him over the phone instead. I just do not understand where this is coming from because he's normally a very well behaved little boy. When I spoke with him on the phone, he was in complete tears and begging me to come pick him up. It broke my heart and brings tears to my eyes just writing about it. He doesn't want to make any friends, and his teacher explained to me that he reaches a point in the day where he just hits a wall, and refuses to cooperate any further. She says he will refuse to talk to anyone and won't sit in his desk... Just flops himself down on the floor and disrupts the rest of the classroom. I am beside myself and don't know where this behavior is coming from. I've had multiple talks with him about it, and have told him that it's ok to have a bad day, or feel scared sometimes, and he can ALWAYS talk to me or his teacher about ANYTHING because we're all here for him and want the very best for him. When I ask him to tell me what's going on, he just shuts down and says he doesn't know why he does it, or says another student was mean to him, even though the teacher assures me no such thing has ever happened. I worry so much about this behavior and feel like if we don't resolve this soon, it's going to start to affect his education further down the road. I was hoping someone could offer some advice as to how I can help him. It's possible he's just adjusting to all these new changes, but so are all the other children in his class, but yet he seems to be the only one acting out. I'm wondering if there is anything I can do with him one on one that will help improve his behavior and help him cope while at school. I'm also wondering if there are any after school programs I can get him involved in that might help as well. From one mother to another, is there anyone who has gone through something similar who could offer some tips or advice? Any help or insight would be so very appreciated! Thanks ladies!