Is the grammar correct?

Assuming the sentence is talking about a patient, it is "... for surgery". No "a".

Also, the sentence is generally clumsy, reading as if by some company middle-manager.

A far tidier and mellifluous form is:

"I have informed him he may require surgery."

Some might prefer "advised" to "informed", as well, in such a situation. The illness may be very serious, but "advised" is gentler. It would depend on the recipient: "informed" would be fine for a fellow-doctor looking after him, but "advised" is better for telling his wife or relatives.

.....


"A surgery" is a room or building used by the doctor, often called a "health centre" if a dedicated building used by a general-practice team. So your original sentence reads as if telling a third-party about your letter to a senior town-planning officer or local politician, suggesting the town's need.

In which case, the tidier version is:

"I have informed him we require a surgery."

You could use simply "need", but in that context "require" gives some gravitas, stressing the importance to the town's population.