Sarah, whenever I see a fairly sudden change in behavior, I suggest a visit from your vet, as sudden behavior changes can be related to physical issues.
If your vet gives your horse a clean bill of health, it may be that you are dealing with some herd hierarchy issues. Hierarchies are normal and healthy when horses are dealing with each other, but a hierarchy that places the horse above the human interacting with him can be dangerous. The higher one perceives himself to be in a hierarchy, the more resources he controls. It sounds as if your horse may perceive himself to be in control of many resources in relation to you.
You can help shift that hierarchy by teaching your horse that he will only receive resources if he behaves in certain ways. I suggest you begin with some simple target training, which will teach him “I do X and I get Y.”
Once he is trained to a target, you may want to do some round pen work—here, you will use your body to move him and reward him. When you face him he should move away. If he moves just one step away, reward by turning your body to the side. If he does not move, make your body a bit bigger by putting your arms out to the side and take a step forward; again, when he moves, turn to the side and drop your arms.
The goal of these exercises is to teach him that you are in control of most resources. Look for opportunities to continue this message. For example, at feeding time, teach him to stand in a particular place, or place his head on a target before he receives food. Good luck!
- Dr. Weiss