S.H.
As Julia and Deanna Leigh said, this happens with toddlers or babies when they have teeth.
Both my kids did that too. According to their Pediatric Dentist, this is NORMAL. Lots of kids do this, and its sort of a phase and it will stop one day. It's not really possible to make them "stop" since they do this as they are sleeping and are unaware of it. I would not interrupt your son's sleep when he does this.. .just let him sleep.
And no, a mouth-guard is not necessary. And at this age, they will not put up with it anyway.
Per our kids Dentist, unless the teeth is getting abnormally ground down or there are other complications... then there is no need to do anything.
Don't worry.
All the best,
Susan