College dorms are a "less is more" kind of place. Don't bring too much stuff "just in case".
Also, make sure that the kid is the one doing the planning, and you're just the helper. I'd help with the big set-up stuff (like sheets and lamps), but I'd let the student worry about little things like laundry detegent and school supplies -- a dorm is a perfect "half-way on your own step", so don't do it all for him.
I agree with the majority of the things that've been said:
Extra-long sheets -- don't forget the mattress pad; that makes a big difference in comfort level.
Small rug for beside the bed.
"Clamp lamps" (that clamp onto a bedpost) are handy for lofted beds, where nightstands are impossible -- all college students need to be able to read in bed.
Extra pillows for comfort while reading in bed.
Milk crates are the best things ever -- we had REAL mik crates, but today they're probably purchased things. Be sure to get the STURDY ones. They're perfect for moving in, then turned on their side they become bookcases, pantries, or nightstands. They can be closet organizers. If you need more space, a couple boards can expand milk crates into larger bookshelves. And then when you're ready to move out, they're moving boxes again. Every college student needs 6-10 of these things. And duct tape to hold them together in the desired positions. Best things ever.
Definitely coordinate with the roommate before bringing any large items. In a small space, you don't need two refrigerators, microwaves, etc. Also, if you're buying a refrigerator, get the small cube type. I had the larger one, and it was harder to move and the extra space wasn't helpful. Even if you have a refrigerator, a small cooler is nice (thinking squishable 6-pack size) so you can bring back a decent amount of ice from the ice machine on the first floor.
I would've loved a washing machine that operated with the student ID instead of quarters!
Check into what's offered on campus before you buy too much. For example, back when I was in school we could "check out" a vaccum cleaner from the security guard in the evenings (he'd hold our school IDs until we brought it back). It cost nothing, and we didn't have to store anything in our small rooms. Also, our health department had a "self care cold clinic". It was a room with posters about symptoms and what kind of meds would be good. No appointment needed, and when you were done, you checked off what you wanted on a list . . . 20 minutes later you'd be walking out the door with a bag of Nyquil, Sudafed, whatever you wanted. No charge. They also gave out free condoms. Well, actually, not free: A chunk of your tuition goes to cover the health department, so you've paid for those services -- so use them! It would be wise to be sure that your student knows that the health department is there, and he should read up on how to access its benefits BEFORE he's sick and needs help. The answers'll be in all that "welcome to campus" stuff, and while the health center information isn't as exciting as how to get your student tickets for the football game, it's necessary.
Also, on that subject, be sure he has an insurance card and knows how to get prescriptions filled, knows what to do in case of health emergencies, etc. Back when I was an RA, you'd be surprised how many freshmen came to my door saying, "My asthma medicine has run out -- I know my mom brings it home in Eckerds bags, but what do I do?" Also, a few freshmen used to expect me to teach them to use the washing machines! It's important that students know these things before they go away to school!
No one's mentioned a good backpack. Maybe it's so basic that no one would forget it.
And for girls, a wristlet-type wallet that'll hold a student ID is handy for days when she doesn't have pockets.
And finally, along with the refrigerator and microwave, think about a few -- few being the key word -- kitchen items. A can opener, a sharp knife and small cutting board, and a bit of dish soap. A couple medium-sized Ziplock containers (for bringing things like dry cereal out of the cafeteria). Also, a couple refillable water bottles will save money and will be good for the environment.