Maybe he needs to bite on something at all times, for sensory issues.

My son was pretty badly self-harming for a few weeks there, everything from slamming his head in walls, to self-strangulation, to hitting himself. It was horrifying. What really worked for us was lots and lots of stimulation like swinging, spinning, deep pressure (heavier than typical therapy) and spicy/or Cold food. We have gotten really good at watching his cues and can usually stave off an episode now by providing more productive/positive stimulation options. My son is a major sensory seeker. His OT from Birth to 3 told me hes the most sensory seeking child she has ever worked with. He needs EXTREME sensory input. I foresee him being into BMX, paintball, wrestling etc later in life. He also enjoys deep knuckle rubs on his back (or back scratches with more pressure than you would expect) I prefer to use a nubby bristle hair brush (sensory brush dont do squat). So long as it doesnt harm him I try to let him experiment on his own. I only intervene and redirect never punish.

I hope that helps, I know how you are feeling and am more than willing to try to come up with some more creative ideas if you need em. We have tried quite a few.

Have you tried the Feingold diet? My son's self injurious behavior decreased immensely once we did this diet, especially when we got rid of Salycilate fruits. His school was commenting on it. We have also seen an increase in cognition with this diet.

Sometimes our little one bangs his head. Recently he started pinching himself and hitting himself on t he face. I try not to read much into those new v behaviors because he started pre school and they could be learned behaviors. He is usually easily redirected to something else.
Biting has been an issue since he found out he could bite. Sometimes we go days without anyone getting bit. This one is hard for us. I was told that because he is NV he has a stronger urge for oral stimulation. We have tried everything. Currently I have a teething necklace. He also has some weaved ball that he chews on.
Last week he got his finger shut in patio door but to my surprise he was easily consoled... That was because the next time I caught him by the patio he was purposely trying to shut his finger in the door.
We also went through a spell of him pulling his eyelashes out. He has a thing for hair. His is pretty long and I have to stop him from chewing it all the time. I have even heard him ripping his little braid from between his teeth.
I just realized that I don't really have an answer for you. I do agree that perhaps it is a sensory issue and getting something he can chew on might help.

Try buying some sensory chewy jewelry for him. You can go on Etsy to look.