Juliet, you gave the answer yourself: he's lost his pal and is in an unfamiliar space. Horses are herd animals and if you take them away from the herd they lose their sense of security! He must be in great distress. Try to find another field companion that is compatible, or move him to a different paddock where he is surrounded by more horses.
how often are you riding him? maybe working him more would help a little. try buying him some toys or balls. keep him busy! 😊
It could be that his first field mate has been at a horse show for 2 weeks and is gone. He is also in a different field than usual. But he has just been acting so crazy lately! I don’t know what to do. Everything scares him.