I used to give my hermit crabs baths all the time. You take a shallow pan of water, not hot or cold, and put some stress coat in it, and let them crawl around in it, but make sure to turn them upside down to get water in their shell too.
If the mites aren't off of him during the first bath then give hime more until you cant' see anymore.
Also Make sure all extra shells, toys, bowls, sponges or anything else in the tank is extremely clean. And if you can't afford new substrate for the tank preheat your oven to 300° Fahrenheit. Take the 'infected' substrate out of the tank and spread it on a cookie sheet to a depth of about ½ inch. Bake it in the oven for ½ hour and allow it to cool. Repeat as many times as necessary until ALL your substrate is sterile. Then clean the whole tank. And if you can vacuum out all of the corners just in case.
Good luck!