What is the ruling on one who says: "I fear that I may disbelieve"? And does he disbelieve by saying this?

Fear of polytheism (shirk) and guarding against it is permissible and مطلوب (desirable/required), as long as it does not lead to obsessive thoughts (waswasah). The Prophet, peace and blessings be upon him, clarified that shirk is more subtle than the creeping of an ant, and he guided to a supplication that removes much of it and little of it: "O Allah, I seek refuge in You from associating partners with You knowingly, and I seek Your forgiveness for what I do not know."