Have you actually seen him chewing it off? I know when my cat had a procedures, the vet shaved a small patch on the inside of her back legs to put the needle in. I not sure if it was for the anesthesia or to take a blood sample.
If you have seen him chewing there, it is possible it still it itches or is irritated from a needle at the vets or the procedure itself. If this is the case, you may need the e-collar until it heals up properly.