DH rides mountain, road and cyclocross bikes, so I asked him for his opinion. If your DS is living in a dorm, most of them have bike storage in their basements so the bike can stay out of the elements when not in use. I would have him check into the storage options that he has before deciding how much money to spend. DH said most of the bikes under $600ish won't haved sealed parts (bearings, etc) so they won't weather the elements very well (could start to have issues in as soon as a few weeks if it's in the elements full time).
What kind of trail riding does your DS do? Is it mountain biking or riding rail trails? Finding an inexpensive single speed rigid bike with disc brakes will have the least maintenence and parts to worry about. Try to buy the bike from a shop in the town where he is going to college so that they are familiar with him and his bike. He's more likely to get better service when he takes his bike in for service than if he bought it from another dealer. Do you know anyone who bikes and could go with him to buy the bike? It's gotten to the point where buying a bike can be intimidating with all of the options that are availble.
If you go to nashbar.com they have a single speed 29 in mtn bike for $379, which would work well for trails and riding around campus.
Giant Rincon is a geared mountain bike w/disc bikes and it's around $500.
The Haro Flightline series w/disc brakes is around $400.
DH suggested seeing if there is a bicycle coop in your area. They might have refurbished bikes for sale. Or even drop a note to any area cycling clubs to see if someone has a used bike in their basement they'd be willing to sell.