Mate I sympathise with you. We told our kids someone hits you hit back. Schools don’t like it but what you going to do can’t please everyone.
My boy is 8 and seems to be in bother all the time. He has been in god knows how many fights. He seems to think it’s his duty to bop anyone who bothers him or his three pals. They are thick as thieves and if it’s not him it’s one of the other three. Kids fight over anything. Last fight was who had the best match attax football card.
My advice speak to the father. Let him know your not happy. If he responds the correct way he’ll apologise and speak to his boy. If not then warn him next time your cracking him.
I might just have been lucky as any dads I’ve spoken to have been brand new and any bother stopped.