That “biting” the fur is normal. I don’t know exactly what that is about, but both Jack and Rucy do that and they do not have fleas or mites. Rabbit scratching their ears is also normal, HOWEVER; since your bunny was found as a stray, both fleas and mites would not be uncommon and should be treated. You should be able to see fleas though, most of the time they will hang out where the bunny can’t get to them, towards the backend. If you brush your bunny’s fur backwards slowly, many times the feces, little visible black dots, of the fleas will be seen even if you can spot the fleas themselves. Fleas can live and procreate inside without much of a problem.
Fur and ear mites though are a different story. They are not easily spottted.
So, since your bunny is under vet care at the moment, just mention this “itching” to the vet. They can give him a quick look-over and scrape for mites. If he does have fleas and/or mites, then it’s easily treated under vet care. (don’t use any over the counter treatments without your vets advice)