Thanks for all the replies. I had not even considered that the need to chew and hum could be sensory needs.
He is sensitive to noise, but not as much as I am, so I never considered that to be an issue until now.
I am going to see the pediatrician next month and will ask for referrals and resources to get an evaluation to understand what is going on with DS.
I will try out the products listed on this thread meantime to see how they help. Thanks again!