R.M.
You don't say how old your son is but is he on any medications? I recently discovered that the Zoloft that I have been taking has jaw clenching as a side effect. I have been cutting down my doseage for the last 10 days and I can already tell a huge difference!!!
You could look into getting a bite guard for your son to wear at night. Ask your dentist about it. I have been told that teeth grinding can injure the roots of your teeth and lead to a serious dental issues.