1/2 of SE is about useless in terms of nutrition. That’s why we’re saying change. Either use a good quality v/m supplement and a cup or 2 of alfalfa pellets as a carrier, or try a ration balancer at a serving appropriate for his size - 1lb if he’s horse-sized, 1/2lb if he’s more pony-like. And even then you may still have to add copper and zinc.
If the water is truly high in iron, then he may need more copper and zinc.
I’m in California and in general, the hay grown here is high in iron which depletes copper and zinc (or something along those lines, I don’t claim to be a chemist or a nutritionist). So a lot of feeds/supplements formulated for the west coast contain higher amounts of copper and zinc and no added iron. The Horsetech product I mentioned (High Point Grass) would be the best for that and you could probably ditch the hoof supplement.
Water high in iron? How high?
Did he shed this year? Is the dull summer coat you’re looking at actually his winter coat? Is it long, with hooked ends? Does he also have a persistent case of scratches, perhaps?
I moved to MN last year and the water out of the well is often pink, it’s so high in iron. Three of my four don’t seem to have a problem, but I have one that didn’t shed this spring and her coat was long, dull, dry and with hooked ends. She’s also has persistent scratches for as long as I’ve had her.
Poly Copper and Poly Zinc from Uckele had her shedding in just a few days, and in several weeks she had a totally new coat. He scratches are also gone.
An imbalance in the iron/copper/zinc ratio can also cause issues with their feet.
newhorse - yes, you had it right 
High Fe inhibits Cu and Zn uptake. High Fe soil means low Cu/Zn forage, for example, which is my case - bright red clay
Ditto all Simkie said! Low cu/zn really messes with a lot of things.
Simke - oh yes, there was shedding. Plenty of it. The water here isn’t too bad. I mean, as long as I keep salt in the softener I drink it. Just comparatively to say, your typical municipal supply full of chlorine and flouride, it has more iron.
Newhorsemommy - none of the local feed stores seem to know what the heck I’m talking about when I ask about that product 
I’ve been researching a few different ration balancers, and I’ve got it narrowed down. I’m going to double check on the cu/zn levels before I try one.
From my experience the more you wash a horse, the more you strip the natural oils from the hair/skin. Especially with a horse that is already lacking those oils. I only use shampoo before a show or when washing a tail. Rarely on the body. however, I do rinse throughly with water and scrape off the excess. The dirt usually comes off with just water.
Also, sun should only dull or lighten color, never the shine. Having a run-in shed should help with the color.
Here you go. Shipping is free.
http://www.horsetech.com/high-point-grass.html
There’s also a version for horses getting mostly alfalfa. And my horses looked great on just a half dose.
