I agree with discussing things over with his owner about his feed. Flax seed is a common ingredient that's very beneficial to the coat. Paprika, as well can help with any sun bleaching.
Otherwise, a new show list product, as well as a daily grooming additive for me is extra virgin coconut oil.
Not greasy at all, gives a very light, healthy looking shine, and over time with regular use softens tremendously. I've only used it about two weeks now, and my geldings tail is comparably softer, and it has a nice little sheen to it. I highly recommend that for regular grooming.
I dropped all other products and have been using just water and coconut oil only, on the mane and tails. It's definitely working, and it's only been two weeks.
You can dye the mane and tail with just regular human dye. Just don't leave it in as long as the directions say. It depends on your horse, but my gelding is fine with just about 6 minutes before I wash the dye out.
For his legs, I use another human product-Roux - Roux Fanci-Full Temporary Color Rinse Black Rage
Do a small bit on the inside on his leg to make sure he doesn't have a reaction to it, and if all is well, just spread it over his lower leg. After the show, you can just wash it off with water. You can also use it on the mane and tail, if you're not comfortable with dying it. I just prefer dye, since it shows up better.