Have him checked for stomach ulcers. Very common in thoroughbreds.
Maybe it's just his personality
He probably is not comfortable with humans what I did with my horse is I sat in his stall with a treat and just let him get used to me, it rlly helped
Sometimes it's just a personality trait. My horse (QHx) is grumpy, but he always has been. I think however, with my horse it could be because he wasn't brought up and trained very well, but sometimes it just the horse. :)
I have a thoroughbred also who will bit when I tack him up. It's just thoroughbreds.