Sorry, I totally disagree with this approach. I would never reward a horse who has made an effort to bite, even if he turns his head away. IMO, you you then teaching him that if he makes the attempt then turns (like playing a game) he will get a treat. Plus, I do NOT hand feed my horses that are mouthy. That is the FIRST thing I take away. It encourages them to be more mouthy rather than less. I also do not agree with not correcting them when they pin their ears, but again, I am more into getting their respect honestly and not through bribery. Would an alpha mare give her foal a treat when they have tried to bite or kick? Not hardly.
OP-you have gotten some good advice, but this is not it. JMHO.
Totally agreed. OP, the advice you have been given is good. Cherie hit it right on the head, he is dominant, and he's making it known.
Do NOT feed him treats, it will only make it worse.
One thing that you said in a previous post seems to allude to the real issue here. You said him in your space wasn't a big problem for you. Here's the thing though, it is a big problem. Horses consistently test their owners/handlers. The reason you believe that this "all of the sudden" happened is because you failed to acknowledge that those problems that are small ALWAYS escalate. So while the problem of biting may be more pressing right now, you NEED to also work on having him out of your space, or it will also escalate.
There are a few signs from a horse that I never ignore. Pinning ears, nipping, invading space, turning hind end. These are all "minor" things to some people, but not to me. I have seen first hand how quickly these small things turn into very big things. The gelding I bought displayed these behaviors because the previous owners babied him. He eventually got over it with me, but it took a lot of work. Now, if he even tries one thing, he gets treated as if it is a big thing.
Stop the behavior when it is tiny, and then you will never have to deal with the big dangerous stuff that comes afterwards.
It sounds like your horse is like mine, give him an inch and he'll take a foot. So don't give him any reason to think he can get away with it.