The only thing about the gray gene is I think it trumps other colors. So a horse could be born any color and then gray out if it inherits the gene.
But that's a good question. Is he brindle and will stay that way, or is the brindling an expression of the gray gene? Obviously most grays never go through a brindle stage, but it could be a strange expression of graying out. I do know however, there are/were solid color brindles with white striping that stay that way.
And some brindles are chimeric, they are actually the product of a reabsorbed twin and so the horse carries two sets of DNA (if I remember my research correctly). That isn't always the case, but it is the case with some brindles.