What temperatures are you used to? How formal do you dress?

Living in the Bay Area, with present weather, and speaking as a rather casual and not-very-cold-sensitive guy... not counting what I'd fly in, 3 short sleeve shirts (polo, button up, whatever he wears), 3 pairs of underwear, 3 pairs of socks, and a second pair of long pants in the bag. Plus a light jacket. Maybe one long sleeve dress shirt and tie if you plan to go out someplace you need one. Should readily all fit in a backpack.

If he's cold-sensitive or you plan to do a ballgame or something involving sitting inactively outside, add a sweater or sweatshirt, which be worn on the plane if chilly, or tied around the waste if it won't fit into the bag if not.

No idea what the women's equivalent; my wife's packing habits are a black art and tend to involve a lot more clothing.