This can be called as nocturnal grinding. One of the reason can be stress. Usually patients aren't aware of this habit as it happens in subconscious mind. If he is a mild bruxer it doesn't affect much but if he is a heavy bruxer then definitely it affects the teeth, jaws and muscles around. In the long run jaw muscles get stiff and starts paining so it's better to control at the start only.
Next Steps
you could visit a nearby dentist and get a night guard made for him. thickness will vary and doctor would suggest you.
Health Tips
during the day time try to be relax and calm.