like I said- if you know anything about a brother or sister etc or his parents I can give you a better idea of genetically what he is
I second (or third, if you will) the opinion that it is max sabino. While most max sabinos cannot be tested, you are very lucky OP. The most common breed that Sabino 1 (the only form of sabino that is testable) has been found in is in the Tennessee Walking Horse. So if you wanted, you COULD test him.
I would test him for extension and sabino first, then if I was really curious about what colour was under the white, I would continue to test from there lol.