Is it a possibility he could be rubbing it? If he is rubbing it, you need to find out why and then address that problem (A common reason is the horse being wormy.) Most of the times if it is spotty, and you can see his skin, he is rubbing it.
Some products to use (Once you find out and address the possible problem) would be Mane N' Tail Grower (MTG) or coconut oil. However, these will just nourish the hair and encourage it to grow, genetics is what will limit the length...
Intermediate Rider~Learning Equitation and English Pleasure~Interested in Eventing
A horse will tell you everything you need to know about him, if you listen.