Totally, completely, and 100% normal, so rest easy! Both of our children (our son, now age 10; and our daughter, now age 5) did the same very disconcerting thing for quite some time. It would give me the heebie-jeebies to be sleeping beside our youngest (we practiced attachment parenting as well) or going in to cover up our oldest and hear this terrible sound and find that they were grinding their teeth! I instantly started to worry that they were stressed out or that they had something wrong with their teeth, but after asking our pediatric dentist I was reassured that this was very normal in children and that it would stop on its own without any damage to the teeth.
Sure enough, our son stopped once his orthodontic treatment started at age 9, and our daughter has already stopped (she just turned 5). I know it sets your own teeth on edge, but no worries, Mom! Your son is ready for his first dental visit, so ask his dentist in order to get professional reassurance face-to-face, but from one who has been there with two kiddos... It's totally normal for children and will indeed stop all by itself. :)