You'd use the same words and body language you would use to keep another human away. Shield the puppy with your palms out flat and use your body to block the other dog, while saying "go away" politely and firmly. Don't raise your voice or make sudden movements, just calmly protect your puppy. Most dogs will pay attention and back off, as you're being defensive not aggressive. Don't worry about anything else except focusing all your body language on maintaining your bubble which only you and your two dogs are allowed into. Do not touch or talk to the puppy except to give him a command or shield him.
Allow him to stick close by your side yet encourage him to be friendly with the dogs around him. He has learned that the world is fun, but not without its dangers and to stay safe he has to be where you can get to him.
Do not allow him to bare his teeth, bark, or treat an approaching dog in a hostile manner - it is not his job to decide who's a threat and who isn't. Reward brave behaviour by saying "good dog". If you reenforce polite friendly behaviour, he'll grow into a very social dog.
Good luck at the park