Do not leave your man hanging without his grooming supplies. Throw everything he uses daily or semi-daily into a dopp kit. This includes a toothbrush, toothpaste, shaving cream, razor, etc. You want bonus points? Throw in a clean set of razor blades.
Shoes take up a ton of room in a suitcase. Pack something that can be compacted down. Canvas sneakers look good with almost everything (even a suit) and can be folded up a bit. If you run out of room in the suitcase, tie them to the hook on the outside of the bag.
Classic polos are perfect for travel. The pique cotton holds up well if you're sitting in a car or on a plane for a few hours and they can be made casual with denim, kind-of-casual with pants or more refined with a jacket.
Let's assume that he's going to want to be able to walk around town, the boardwalk or wherever else you're heading in comfort. Throw in a pair of chinos and he'll be set. They'll match pretty much anything he owns.
A chambray or denim shirt is a great piece to travel with. It's light, comfortable and wears well on virtually any skin tone. Plus, it's a good option for wearing with swim trunks. It'll give him an elevated poolside look.
A deconstructed sport coat packs easily and has less of a chance of coming out of the bag wrinkled. If you're planning on going to a fancy dinner, a country club or any place where a jacket is required, he can throw this on over his polo or denim shirt. Done and done.