My kiddos is sensory seeking w his mouth...it ebbs and flows...we have Zvibe and different chewy necklaces and rings...also work w his OT on different strategies....crunchy snacks, etc. I have heard of something called Pica which you could ask the pediatrician about, that has a nutritional component if thats of concern!

Have the doctor draw blood to see what vitamins are missing.

It is common for children with ASD to have oral fixations. Mine has improved (decreased the licking) but when anxious he wants to chew. They do make oral chew devices specifically for that.
I keep rope (non-toxic, clothe) at home and tie knots in it for him to quiet his busy hands and chew on when extremely anxious. It can be put in the washing machine easily and reused...plus if he throws it, nothing breaks.
(while I would rather him not use the plastic chew devices...at school they require something that can be cleaned easily.)