You definitely need age, height, temperament, is he registered, registered name, any show experience, any show accomplishments, how much training he's had, more pictures, vices, location, and contact info and a price! :) Posted via Mobile Device
One thing you should mention is his movement. Examples: He has a forward trot with lovely reach. Uphill in his gaits, balanced canter, forward walk, describe his gaits. Doing so will help people understand the type of horse he is better and that way when prospective buyers come to see him, there are better chances they really are interested.
Also don't false advertise. If he has bad vices, be honest but phrase it correctly. Make sure everything you say is true about him so he's the horse the people expect him to be.