Okay, so being a brindle he could be Chimera (two embryos, two sets of genetics, merge into one foal that has two sets of genes... so if you DNA tested him, he's have two sets of results). Little is known about the brindle gene though. So in a nutshell if both sets of DNA have the grey gene, he will grey out. However, if only one set of DNA has the grey gene, it will continue to grey out while the other will stay its own color. That is, if the Chimera thing is correct.
It looks to me like he's going to grey out, or at least be reduced to flea bitten grey. I say this because of the way the darker color appears kind of like a rose grey as someone said before. I don't know though, only time will tell!