Absolutely report it. Include the vet's bill - which should have a write-up of his injuries, and any pictures you can get of poor Gonzo's injuries. This man should be banned from the Dog Park, which is NOT going to happen if no one reports the abuse! And as a plus, he should be made to pay your vet bill, but that is secondary to ensuring that this never happens again.

Poor Gonzo! Give him a snuggle and a tummy rub for me, and then another snuggle and tell him he did the right thing. Poor Gonzo, and poor you!