I've got a 5 year old thoroughbred gelding and he bites etc and always is grumpy! Why is he like this?

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.
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