No dog was ever ruined by holding it back, but plenty have been ruined by letting them start too early. Personally I would wait until September, whats the rush, it's summer, let the pup be a pup and work on the other aspects of his training. If he runs a dozen and sees them disappearing he will soon start to pull up, not chase or develop a myriad of problems and in three months time you'll be starting a thread "Why won't my pup chase?" or something similar.
Not quite sure why everybody is in such a rush to get dogs going, it's almost summer anyway.........