I am going to disagree with everyone that is saying bay roan. He is showing classic signs of being brown, and a brown roan at that.
Just because he's dark in winter does not make him brown - he is BLATANTLY gold colored in summer, and if he were a brown roan, then he should be showing as a blue roan and not a buckskin/dun color. There is nothing in either black or agouti genes that would make him golden in summer. My fading black may turn brown, but she certainly isn't ever going to display such clearly golden coloring.
Here is an example of a buckskin roan, showing exactly what this horse is displaying - blue roan colored body parts with blatant gold on the face and highlights: