I would probably say chesnut. It doesn't look to me like he really has a defined dorsal stripe, I think it may just be the angle or what have you. He looks more chesnut than sorrel to me. My bs paint mare "Darlin" is sorrel. She has way more red/orange than you little guy has. I would have to say chesnut. Here is a pic of my sorrel mare just for comparison. Its not the best picture, but you can see the color really well.