Mane N Tail Shampoo has always worked for me, but I LOVE to use Dove Shampoo on mine....makes them smell good even when they sweat or get wet (rain).
If you are having trouble getting "stains" out of his coat you might can try a whitening shampoo like White N Brite Shampoo
It has always worked for me on light colored horses....just don't leave it on too long, you may end up with a purple tinted colored horse (this has also happened to me)
I would also recommend these supplements, all of which I have used in the past and present that I received great results from. Gold As Sun by Cheval International
Or SmartShine by SmartPak
/ SmartShine Ultra by SmartPak
Nu-Image by Select The Best
Grand Coat by Grand Meadows
I think my all time favorite has been the Nu-Image by Select The Best. I have seen great results from it and my mare (very picky eater) loves it.
If I had a Palomino I would def go with Gold As Sun, the Palominos I have fed this to in the past had all gotten a lot more golden while being on this supplement.