Ashley, does your ds have any other signs of sensory needs or any type of anxiety? Or have you tried asking him if he is distracted by anything when he is working at focusing while doing homework etc? It's possible the chewing is simply a need to chew, but sometimes sensory-type habits are prompted by something else, and if you can find that something else you can figure out either a way around or a way to get past the habit such as chewing.
My dd11 is a sensory-seeking child, and she used to chew when she was around the age of your ds. One of the things that was suggested to us to use for school/homework were pencil "erasers" that were intended to be chewed on - they look like fun kid-erasers, the type you stick on the end of a pencil, but they aren't really erasers, they are chew-things (sorry I have no idea what the technical term is or brand name!). We were able to order them through one of the companies that sells sensory-integration therapy equipment etc - I can't remember which company specifically, but I think there was a company named "Southpaw" that we ordered her a seat cushion through, and they might have something similar.
What really helped my dd rather than replacing what she chewed on etc, was finding ways to reduce her need for the sensory input. One thing that helped her a ton was brushing - this isn't something you're supposed to do without being shown how to do it first by a sensory OT, but it's essentially brushing gently on the arms and legs, and then gently pulling in and out on the joints of the arms and legs. Again - this is something that you shouldn't ever just try to do on your own, you need to see how it's done first, demonstrated by a professional who knows what they are doing
But, fwiw, it felt good and it really helped calm our dd's needs for the other types of sensory input. DD also had trouble focusing when there was noise in the background (either at home or in the classroom) and so she went through a listening program which helped out there - and once she was able to more easily tune out background noise, we saw a big reduction in her sensory-seeking behaviors.
I suspect what's going on with your ds isn't anywhere near as complicated as my dd's sensory needs, but fwiw, if the chewing is just really bugging you or if it's causing social issues for your ds, I think that even just one appointment with a sensory integration OT could give you some valuable advice about how to deal with it for your ds specifically.
Best wishes,
polarbear
ps - another thought - if it's mostly a habit and not a sensory need - your ds is at an age where you *might* see worries about what peers think kick in. I loved loved loved to suck my thumb when I was young and wasn't going to give it up no matter what - until I was in first grade and started being aware that I was the only kid who was still sucking their thumb. Do you know if he chews while he's at school in the classroom?