Well, I read somewhere that brushing a horses mane as sparsely as possible might help, but I don't think it will really "grow" the hair, so I question the reliability of that. Otherwise you might be able to find a shampoo that could help the thickness of the mane, I've heard of such things but never tried them myself.
Then again it could just be the breed/breeding of the horse. Horses bred for work in mountains would have a thicker, sometimes longer mane than a horse bred for riding in a more warmer climate. And horses bred in the desert sometimes have longer manes, most likely to keep sand or flies from their eyes (I'm assuming.) But you might see some that have little thin manes. It usually depends on the breed and the area most of the horses were bred from.
I have a POA that has a short mane, but it's very thick and lays forward much like an "emo" kid brushes there hair. (Yeah, that's the only comarison I have. XD) Then I have a Paint with a very long mane, but it's very thin. (It looks alot like your gelding's mane, the same color, too. :3) The POA was born in Missouri, and the Paint in Wisconsin. I really depends on the horse, because the more I try and explain it I just get more confused.