Well he isn't a leopard. He'll get whiter and whiter every year, but it is not grey. That is the varnish (LP gene) at work. Like I said, I bet he was born with a blanket and has varnished to this point. *shrugs* Vets usually aren't the most up to date on horse color genetics.
His spots he will keep if they haven't varnished yet. Varnish tends to not affect spots (grey erases them). It is more apt to reveal more spots than it is to erase them.