Look on the back of his registration papers; it should have his pedigree back at least 4 to 5 generations. His parents' names will be on there with a letter designation. G is gray, C is chestnut, B is bay, etc.
The reason he's got the whole black and brown thing going on is because gray isn't a base color, it's a modifier. All gray horses are born another color; chestnut, black, bay, palomino, buckskin, etc. As they age, the gray gene kind of takes over and causes them to go gray...and eventually, white.
Yes, it looks like he was born black, and the grey slowly comes it and starts to leach the black away. So black can look lighter and brownish. They can even go through a period when they look like they are buckskin or dunnish. They can also sunbleach.