He just needs to marry and still be married 3 years later when he files his N-400 and continues to be living in his marital union with his USC spouse all the way through Oath and be otherwise eligible.
INA 319. [8 U.S.C. 1430]
(a) Any person whose spouse is a citizen of the United States,
or any person who obtained status as a lawful permanent resident by reason of his or her status as a spouse or child of a United States citizen who battered him or her or subjected him or her to extreme cruelty, [VAWA, not you.]
may be naturalized upon compliance with all the requirements of this title except the provisions of paragraph (1) of section 316(a) {This includes by reference, the time from filing through Oath for USC spouse's status, marital union, "continuous residence", and good moral charcter requirement and shortens the period to 3 years instead of 5 years.}
if such person immediately preceding the date of filing his application for naturalization has resided continuously, after being lawfully admitted for permanent residence, within the United States for at least three years, and {This means don't set up house or take "unqualified employment" abroad or take really long trips.}
during the three years immediately preceding the date of filing his application has been living in marital union with the citizen spouse
(except in the case of a person who has been battered or subjected to extreme cruelty by a United States citizen spouse or parent), [VAWA, not you.]
who has been a United States citizen during all of such period,
and has been physically present in the United States for periods totaling at least half of that time and has resided within the State or the district of the Service in the United States in which the applicant filed his application for at least three months.