Very good question. Either get him a black, classic cut suit, one white shirt and 2 ties, or rent a tux for formal nights. Every man should own a dark suit, there are times when it is needed, no matter how casual his normal life is.
The rest of the nights, 2 pair of slacks (Dockers or khakis) and 3 sport shirts, you can have them laundered onboard. No one will notice or care that he wears the same clothing twice. A pair of black shoes and a pair of deck shoes. 3 pair of shorts, 4 Ts and a couple bathing suits, underwear and socks, he's done. Most people pack waaaay too much, especially on their first cruise. Using the laundry service onbaord is a whole lot cheaper that buying shirts and pants he's probably not going to wear very often.
My DH wears the same outfit the first and last night (I usually do, too). The more we cruise, the less we pack, the easier it is to travel and organize our cabin for the week.