My first reaction as a recovering redneck is to simply shoot the dog if it menaces me. If you want to be nice, you tell him first to put the dog in the truck before you shoot it. You'd be safe legally if you can show that the dog has bit people before. But that's a little over the top, so I'd suggest the following.

Bring a package of hamburger. Walk by the guy, tell him that you don't appreciate his dog running around and that there is rat poison in the hamburger. Lay the hamburger on the ground next to where you are fishing, making sure that he sees it.

It's his responsibility to contain his dog, not yours to keep his dog from getting in trouble.

I say this as a dog owner who takes his dog fishing. I like to have my dog along, but I make sure that she doesn't pester people, let alone menace them.