Your rabbit might have ear canker, which is caused by small parasites called ear mites (Psoroptes cuniculi). Check his ears for a brown discharge and crustiness. Ear canker can easily spread from one rabbit to another, so if you have other rabbits, check their ears also. Ear canker can be treated and cured by using chemical mitacide eardrops safe for use in rabbits or an injection obtained from your veterinarian. A few drops of light mineral oil used as eardrops can also be an effective treatment. Mineral oil should be applied with a cotton swab.
Consult your veterinarian about the best treatment for your rabbit.