He's just old enough to visit a pediatric dentist. I'd take him in for a visit to make sure there is no problem with any teeth he might be cutting. Children of that age group tend to chew on crackers when they are teething, especially when there is a tooth that should be breaking through but is taking it's slow time at getting through the gum.
If the dentist doesn't find any problems, then perhaps it's time to take away crackers completely for a short while until he's eating a better more diverse diet then slowly and gradually begin to offer crackers back into the diet.
I had similar concerns and still do even though my girls are 23yo and 13yo because their dad is a juvenile diabetic. I've had both tested every year since they were born because contrary to what many doctors, including pediatricians will tell you, diabetes does tend to run in families. Also, in a medical journal I read not all that long ago, diabetes is being reclassified as an autoimmune disease being that it acts like one in every sense of the word. I have lupus (SLE) and Sjogren's disease and with hubby having juvenile diabetes, we worry. My youngest has already been tested for a variety of autoimmune diseases just to establish a baseline because I am of Native American Indian heritage and autoimmune diseases tend to run in Native American, Asian, and African descended families. I have many family members on both maternal and paternal sides of my family with a variety of types of autoimmune diseases. I was the 2nd of my generation that tested positive. The one before me has juvenile RA. Now, there are several cousins who are just now or have been for the last couple of years become symptomatic of autoimmune disease so they are testing or have been diagnosed and are on treatment plans to help slow the progression.