Hello, I have a problem. My eldest son has been grinding his teeth for…

Hello, I have a problem. My eldest son has been grinding his teeth for about a year, then this fact passed and now I notice these grinding again. (The grinding is always only at night or in the afternoon during peak sleep). What's causing it? Is it dangerous? And which doctor should I see? (When we went through the commission at school they didn't notice anything unusual.) We also did an analysis for intestinal infestations (worms), everything was clean. Or maybe we don't need any doctor at all.