I would start with a visit to his pediatrician, who can then refer your grandson for testing if that is needed.

The earlier you address this problem, the better. If it is just a matter of habit (unlikely) then he can be redirected while he is still young and that will hopefully eliminate the problem.

If there is a medical cause, then therapy and/or solutions can be undertaken at the earliest opportunity so as to leave your grandson with as few disadvantages as possible.