Chestnut with flaxen , sabino, and probably frame.
Flaxen is causing the lighter colored mane & tail. This is likely why some people mistake him for a palomino, but to be a palomino he'd need to have the cream gene which would make his body color much lighter. He'd also have to have a parent that carried the cream gene (palomino, buckskin, cremello, etc.) for this to even be a possibility (not sure if he does or not- just pointing it out)
Sabino and frame are causing the white pattern. Sabino causes the jagged white and "roan" (which is not the same as true roan). Frame causes the white to move in a horizontal fashion, which you see most obviously on his neck. The APHA lumps all of the non-tobiano white patterns into the term "overo" so calling him a chestnut (or sorrel) overo is a simple way of describing him, but not very precise. As a gelding, it's not as important, but for breeding horses it's very important to distinguish frame from the rest of the overo patterns because it is the cause of Lethal White Overo.