Abby, It's strange that his mane is coming out but not his tail. I agree that a vet may be in order.
A few other questions come up that may have nothing to do with it but ... why are you feeding him so much? Is he a hard keeper? Do you ride him that hard every day? My performance horse gets ridden every day for at least 1/2 an hour and several times per week for several hours. He is turned out 24/7 but with the pasture not growing I have to give him hay. He gets 2lb of 10% protein feed that contains all his vitamins and minerals twice per day and that is all he gets. He is not exactly an easy keeper but close. This horse is used for team penning, trail riding, and some roping. He stands about 14.3 - 15h and weighs in at ~1,150 lb.
Secondly, how far up on his withers are you placing his saddle? Does his mane come down that far? When I saddle up only about an inch of mane is under the pad and when I start to ride, even that will become exposed.