Teaching him leave it or gentle/be nice are great ideas.
Gunther was the same way when he was younger. At our dog park, I would simply tell people he was still a puppy and loved little dogs, and that usually sufficed for most people. Most owners should understand puppy play, and unless they have a chihuahua or something similar (where he can stick the whole dog in his mouth......... lol), I think the little dog should be able to hold it's own.
Just to let you know, they usually outgrow this stage. Gunther now plays with dogs of all sizes, and now prefers medium to medium-large dogs, although his best friend is still a basset hound.