Bass Pro Shop. They have a very nice variety of zip-off styles that are meant for outdoor wear, and their prices are very reasonable. Zip-offs will be more useful to him in the long run because he will also be able to wear them in summer. Their Angler pants are $20 right now, and come in a nice travel-friendly charcoal grey.
http://www.basspro.com/webapp/wcs/s...1_97425_600005004_600000000_600005000_600-5-4
For the long underwear, he should have silk or hi-tech wicking synthetic. Don't defeat all the effort on the trousers by sending him with cotton long underwear that would also take forever to dry (and not be nearly as comfortable, either.) You'll probably end up paying a bit more for these than for the trousers, but he'll probably only need one pair.
Don't forget about his regular underwear and socks. Tighty-whities and cotton socks also take forever to dry, so if he doesn't own them already you should buy him a few pairs of sports underwear and runner's socks that will dry quickly when washed in a sink.
For shoes, I would suggest Merrell Jungle Mocs, or the Bass Pro knockoff versions of them, which are fairly decent for lower-priced shoes. They are water resistant but work even for slightly dressier situations.
Also, one last tip. Send him with a long nylon cord he can use for a clothesline, and also a flat rubber jar opener disk, which can be used as a sink stopper when the stopper is missing, which it often is.