It wont hurt to take him along to a dentist - even if they do nothing, it still gets him used to the chair and surrounds for when you do have to take him for treatment kwim? The more familiar he is with it the better in the long run.

As for the grinding, my three did it from time to time and if I caught them when they did it, I would tell them no, stop grinding your teeth. I have no idea if it worked or not though.