J.G.
I have this w/both my kids (one in kindergarten one in 3rd grade--it's sooo fun when they both go at it and are both grouchy :(). I think they're tired of holding it together all day and they can let down at home. I found out w/my older that she wasn't eating good things at lunch so by the time she got home her blood sugar was low and she was a mess. Talk w/his teacher to make sure he's eating what he should. Try giving him a snack right away and then letting him chill out by watching tv or doing something he enjoys. Make sure he's getting enough sleep at night so he can get through the day. I do think all day K is tough for some and it's a huge adjustment so he's probably just dealing with a big change. Good luck!