Hi, Lynn. Well, the problem is that we see this kind of prank all the time on TV. America's Funniest Videos has at least half a dozen of them every episode. Everybody laughs hysterically when this prank is shown. So your sweet son has seen this on TV, no doubt, and believes it's OK because no one punishes anyone for this kind of prank. He probably didn't know anything was wrong with it.
So I would start there with re-educating him. I wouldn't panic -- he doesn't sound like a bully or a mean little kid at all. Also, 5-year-olds don't know their own strength, so he probably doesn't understand that he could've hurt the other kid by slamming his head into his plate.
I'm sure it looks worse than it really is. However, I would make sure he understands that this kind of prank is mean, and it's wrong to laugh at someone getting treated this way. Maybe you should keep him from watching that kind of show where people play pranks on each other and get praised for it. I don't watch that show very much myself because I really don't like to laugh at other people's pain. Call me a weirdo, but I really don't.
I hope this is helpful.
Peace,
Syl