How is it best to deal with nastiness in the playground

We went to Looe, Cornwall with our three children last weekend. It is a fabulous site with pool, tennis court, city golf, large playground and ball play area.  However, our son (9, almost 10) came back quite traumatised. He asked a group of boys aged 12ish if he could join in their game of football and was verbally battered by way of reply. The language was astonishing, the f word, the c word etc. for the rest of the weekend he was afraid to go out to play and was even afraid to go to the toilet block without dad for support In case he bumped into the boys again.

How do you deal with that? Advise please.