My daughter did the same thing at 14 months old. It's probably because they're just learning that they can make cool noises with those little white things growing in their mouth. My daughter grinded her teeth constantly, it drove me nuts, I hate the sound of it. But I relized that the more I tried to get her to stop the more she did it. So, we tried ignoring it. It's not easy but it works. It may take awhile for him to realize that it's not getting your attention anymore. But if you ignore him when he does it eventually he will stop. It also helps if you get him into something like a favorite toy or book. And these are just his baby teeth. He can't really do any damage to his permanent teeth yet so I wouldn't worry about it making his teeth crooked. Well, good luck. Hope I helped.
Oh, and if it gets too bad and you just can't ignore him anymore, give him a snack. He can't grind his teeth if he's munching on something. My daughter loves the goldfish crackers. And you can get them in cool colors. and I dont mean stuff him with snacks all the time but it works if he's been grinding his teeth for awhile and it's starting to drive ya crazy.