I was going to suggest as Karen did. Even a couple good sizes rocks. Be sure they are too large for him to swallow, and wash them in hot water to have them clean. If he has to hunt for his food, it is more natural as well as slowing him down.

Feed him dry kibble once a day, and wet food once a day. For the wet, you may have to just put down a dollop, wait 15 minutes, put down more. It's a nuisance but worth it after a week or so, he should catch on that food is always coming, and slow down.