I have the exact same situation with a child who has Down Syndrome. I tried providing him with a Chewy Tube, which he successfully used, however his mother reported that she noticed he began to put his hands in his mouth more often and felt this was a worse result (germs). I think it is most likely sensory related, so I usually try to give him some type of manipulative to keep in his hands (i.e, a squishy ball). I also use positive reinforcement for "quite teeth" as often as I can. I recommended that the family ask their dentist for some input as well, as the grinding is very damaging to the teeth.
Thank you for sharing!