Sorry to hear this, I don't why people grind thier teeth. But I can say I found out that I do. I changed denists many years ago and he noticed it by looking at my teeth. Grinding can do alot of damage. He prescibed a night gaurd that goes over your teeth at night. It's not uncomfortable. When I first started wearing it the headaces I was having went away, just as my denist said they would. Now when I forget to put the night gaurd in I can feel it in my jaws, teeth, neck, etc. So it will be a good thing for you to take him to the denist and check out the possibility of getting a night gaurd. My guess is this is where we take out the stress of life. Not sure what a 5 yr old would stress about, but you never know, the little ones worry about things we never think of. I have recently noticed my 3 yr old grinding at night some times as well, so maybe it's inhereted?