It seems like common sense, but you would not want to put a powder in his ear; that could exacerbate the problem. DE is not effective for treating ectoparasites anyway. Try examining him over, and see if mites are the problem. If you don't see any ectoparasites, check the ears well, and look for signs of gunk, indicating ear canker, or bacterial infection.
If you see mites, you can buy a permethrin-livestock spray, dilute down with water per directions. and spray around under the feathers, around his neck, underwings, and near the vent. If you see any by the ear, you soak a cotton swab with dilute permethrin, and swab around there, not getting inside the ear.