If his coat is already bleached, you will have to accept that he will be that colour until his next shed.
How old is this colt? I am hesitant to start filling a young horse up with supplements for something so trivial as coat colour. The old saying "a good horse is never a bad colour" doesn't just refer to not judging a horse only on it's colour. It also is because a healthy horse on a well balanced diet is usually going to be a good example of it's colour. Rather than supplement him, balance his diet first. If you are worried about him bleaching, either rug him or turn him out during the night and keep him in during the day.
It's just like people - multivitamins should never take the place of a balanced diet. Why should they in your horse?
It is too hot in the summer to stall a horse or to put rugs on, sun sheets tear up easy and are expensive. I have a palomino, buckskin, bays, and I feed Sun flower seeds in their oats once a day, this will produce a wonderful shine to their coat, helps strength the hooves, and will also also add some good fat to the diet. I like the seeds better than the oil as it is too messy for me. I also started with the "black as Knight" this spring to my palomino, and WOW what a golden color he is this year, everyone that stops in notice what great color he has this year compared to the sun bleach pale he has been before. He is total turn outside, My friend also uses this product for her black horse, he is somewhat better but still has some red tones in his coat.