Some horses loose their bloom over the winter, when it's harder to find good quality forage. My advice to you is to be certain that he's getting the best quality hay possible, pasture as the grass comes back, and perhaps reassess his grain ration (if applicable) and make sure that his nutrient intake is well balanced. A shiny coat comes from the inside out, and no amount of shine in a can can replace good nutrition and condition. Fat does not equal excellent nutrition/condition. Also, check into his deworming schedule. A worm load can produce a duller coat. His teeth may be the culprit as well. If the horse isn't chewing efficiently, nutrients aren't absorbed well.
If minor diet changes and increased grass intake as spring progresses don't do the job, you might look into a feed-through coat supplement. However, I've never met a horse yet who couldn't be brought back from a dull coat with minor feed adjustments, regular and correct deworming, and a daily dose of elbow grease grooming.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown