Nails falling off?

My son is 21 months old and his two big toe nails are broken at the cuticle all the way to the bed, it is not just the top layer of his nail. One nail it goes all the way across and the other its about halfway. He hasn't had any trauma to the nail, any sickness, no HFMd and nothing that I could think to cause this. He is very healthy and doesn't act like it hurts but literally if the middle of his nail wasn't attached to the nail bed I could just take his nail off. Any ideas what this could be?