There are a few things that can cause what you are seeing. The first and most alarming is that he might have ingested a toxin. If you even remotely suspect this, you need to rush him to your vet right away.
If the eyes are vibrating or going back and forth like they are watching a tennis match, your cat might have vestibular disease, a disease of the inner ear. It can cause stumbling, falling over and a general discoordination.
No matter what, you need to go ahead and see the vet with him, particularly if he is not eating well.
Let me know if you still have questions.