As his hair is already damaged, you want to keep the shampooing to a minimum. Good nutrition is the biggest help but also, vacuuming the area to remove dead skin ie. Dandruff. A mix of 50% Baby oil and 50% regular listerine applied weekly and rubbed into the base of the mane will help too. Try to not to brush it too much, give it a chance to grow without pulling it out. If it gets too gunky, then you can wash but use a horse shampoo, and go easy on the rubbing. Rather gently scrunch the mane with the soappy water to wash and gently rinse. Use a good mane and tail conditioner after washing.