Head tilt is a situation where the bunny doesn’t know which way is “up” and in consequence, he tilts his head any number of degrees in an attempt to right himself. In extreme cases, the rabbit rolls over and over in a futile attempt to straighten up. It’s very sad to see.
Head tilt can be caused by neurological disorders, a middle or inner ear problem, stroke, trauma, or e. Cuniculi, among other things. If diagnosed properly and treated quickly, it can resolve itself, but often the rabbit will live out his days with a wry neck.
If your bunny is starting to hold his head at an odd angle or is circling in one direction, take him to a good rabbit vet. There may be a way to make him better. In the meantime, you might try rolling towels with which you can prop up your little guy against the wall of his pen or box. His “down” eye will probably become infected at some point, so you and your vet should watch it for problems, as well.