If a child hits their head on the wall or floor it is time to act.
Maybe ignore the hand stunt. When you do that a few times he may get bored from not getting any response. If he sees his grandparents on a regluar basis and he does it in front of them, ask them if they can do the same thing otherwise it may slow the behaviour you desire.

Yep, as said before we had to ignore and distract our kids who hit themselves in the head as well. I’m not sure why it’s so hilarious for them?!

We have had the same problem with our toddler who hits and bites himself. We find that ignoring this behaviour has helped.

with kids it is ‘stages not ages’ he is getting attention so he will keep doing it. ignore the bad behaviour and reward the good

I wonder if it just boredom . My son used to twitch his nose especially while watching TV , and it did eventually stop. Maybe just distract him so he finds something more interesting to do, like looking at a picture book etc..

try to distract him instead. i know that kids can pick this up off their tv shows lol. he will get over it soon.

Apparently this is not an uncommon habit and many a parent has been faced with the problem. Google has many sites to view and some have doctors responses.
Best of luck.

Sometimes ignoring such behaviour is better, combined with other distractions (but don’t reward to bahaviour!!). That is assuming he is enjoying the attention received – even a telling off is attention.