Is there any particular area you are grooming when he does this?
If he just does it in general, he's probably impatient. Hoover does the paw and the head toss when being trimmed, because he's my ADHD horse
. I firmly tell him no, and take a pinch of his neck skin and twist it if he doesn't listen. The minute he stops, I release the skin.
As for the kicking, make sure you discipline him when he raises his leg, before he kicks, if you can catch it.
And everyone's absolutely right...do not stop grooming him unless you are in physical danger and have to jump out of the way.