Thanks, Lauren and Mama Nancy.
Well, this behavior just started late last week, and we just found out that his medication level is a little too low, as he has started feeling seizure-prone again for the first time in a long time.
So, maybe that's why.
We have talked with his pediatrician and neurologist a lot about his behavior, previously (not the hitting, cause that only just started). They were very helpful, saying his behavior was pretty normal for a little boy just starting kindergarten (and he only just turned 5 in October). His neurologist is the one who suggested a start chart/rewards system, and his teacher implemented this shortly after school started in the Fall; this has been very successful. His behavior has gotten much better. But now, this hitting thing.
When we'd ask him why he's hitting others, he says because he's angry. When we ask why he's angry, he says he doesn't know.