Didn't realize that there's actually a company that makes bite-resistant clothing. Expensive stuff - $250 for a shirt. But if it lasts, it might be feasible.Bite-Pro
The short sleeve version is over $180 + shipping. That's not $250, but it's still a lot of money for an experiment. I have a feeling they would hold up well to the occasional bite but not constant chewing. No fabric will. So at that price, it's not worth trying (and even if it was worth a shot, I still can't afford it).
So, I think really the best option is going to be for us to always have something sacrificial that he's allowed to chew handy for him and teach him to use that instead of his shirt. This may be a battle. He knows he's not supposed to chew his shirts, but he can be pretty willful.
Out of pure interest, I looked up the fabric that bite-resistant clothing is made of, Cut-Tex® PRO, and I see it's a blend made largely of UHMWPE, so at least my thinking was on track.