If you ask them what other breeds they have, you might be able to guess what mix he is. I have two roosters that appear nearly identical. At first I assumed the younger one was a Cream Legbar like his daddy. It was a safe assumption since I had collected only pure blue fertilized eggs at the time.
However, the new roo lacks barring that is typical for Legbars. I had likely picked up an egg from one of my EE hens that lays pure blue eggs, so I can conclude he is a Legbar/EE mix. I looked up a photo of an EE rooster, and he's identical in color and pattern.
Yours could be a RIR/White Leghorn or White Rock mix if you find out there are Leghorns or White Rocks in the flock where he came from.