That's a hard one. Do you know if his gaits at all?
He almost looks like he may have Tennessee Walker in him
My other guesses would be Thoroughbred X QH or possibly a Belgian crossed with something much lighter
I doubt he has Walker in him to be honest. Walkers are not too common here in Australia. He looks like a QH to me. I can see that he is branded, have you tried tracing his brands through the various breed societies? That is probably your best bet to find out what he is, rather than guesses on here.