How to know if you horse is going to gray out...?

I have an arab cross with unknown lines and unknown parents. He's a rescue and is 2 years old approximately. When we took him on he was black and has steadily been getting lighter into tan colors like a typical bleaching out dark bay horse but I've had a few people say he will gray out. It doesn't matter to me either way. When will he start showing if he's going to turn out to be gray or not? Wouldn't he already be showing signs of graying out? He's got some white spots on his withers but that's it. When he gets bitten in the pasture, the hair grows back dark black.