I wouldn't worry about being disrespectful by not wearing a jacket. I'd guess that even the professors will not be wearing them. (Although you may get one or two eclectic bow-tie-and-tweed-jacket types, but they certainly don't expect anyone else to dress like them.)
Any color shirt. I'd probably avoid really loud patterns and things, but otherwise it doesn't matter. A nice sweater over it would be fine too. If he really wants to wear a jacket, I'd probably go for something like a tweed sportcoat over khakis, although as Greyscale mentioned, he may look out of place in the jacket.
ETA: shoes. Depends on the program/department, but at bigger places there will probably be a lot of schlepping around to see different professors/labs/buildings and possibly a campus tour or similar (so outdoors), not to mention going out for dinner later. So he should make sure he wears comfy and weather-appropriate shoes, although this is of course a much bigger issue for women.