Does he have access to free choice minerals? Not just a salt block either...but minerals? I know a lack in certain minerals can cause an improper shedding pattern and dull coat. In a dark colored horse a lack in copper can cause a copper colored mane and tail to come in, as well as a dull hair coat.
He does appear bright eyed though, and his coat doesn't appear 'dry, or brittle' by anymeans....maybe just spend some time really grooming on him, and that may bring out the sheen in his coat; he may just not be fully shed out yet either, as it is still early in the 'spring' yet. Two of my horses aren't even done shedding yet.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."