Eyes are delicate & can get infected easily. I think having a vet remove the mite would be a good idea...from your description it sounds like it's really entrenched?
The only other thing I can think of is a very carefully-directed application of mildly-soapy* water. (*Ivory or puppy-shampoo): you mentioned bathing him but was
that just in plain water? FYI, bathing a snake in soapy water* (luke-warm of course) will get rid of most mites on the snake's body by drowning, if you patiently
stay with the snake for 20-30 minutes. (shallow water, the snake shouldn't have to be swimming to stay above water!) The soap breaks the surface tension in
the water so the mites drown. Now IF that mite by his eye is truly the only one, soapy water shouldn't hurt his eye (it's covered by a scale) & you might try using
maybe a Q-tip saturated with soapy water directly on that mite...see if you can make it expire or let go? I'd personally try this before running to the vet, & then,
it wouldn't hurt to give the whole snake a bath too, just in case. But do the eye separately, you must not immerse their head nor allow them to drink soapy water
...do not leave him alone. This is the low-tech way to get rid of many or most mites, but they can still hide on the head, so it's not 100%. It's a good emergency
treatment: say if you got a snake that had lots of mites & no chemicals to treat immediately, you'd want to get rid of as many as you could to minimize blood loss-
mites can and do kill snakes, even though it doesn't seem possible because they're so tiny. (don't under-estimate mites)