That's what he looks like when he's wet, just with the pink showing through his wet hair
Colored horses (black, palomino, chestnut, etc.) have black skin. Add the grey gene that turns the hair white, the skin will still be black.
Patterned horses will have a mixture of black and pink skin, pink only where the white patterning is. This is also true for any generic white markings (star, sock, blaze, etc.), as they are also caused by the white patterning genes.
Your horse most likely started out life as a whatever-color pinto that greyed out, thus leaving him with all white hair (but the skin color doesn't change so you can still see what his pattern looked like underneath all that white). Depending on what his base color is, if he was always considered all white even as a foal he could possibly have been very light-colored.