1)Bedding -
Buy the best, thickest memory foam mattress pad. Also buy extra deep pocket sheets so that the memory pad can fit underneath it. Along with comforter, also give fleece and cotton blanket to make the bed extra cosy. Good quality pillows, pillow protectors and pillow covers. Buy a "body rest" pillow too. Make a headboard if needed. Check out this tutorial -
http://theoldpostroadblog.blogspot.com/2013/06/easy-dorm-room-headboard-tutorial.html
2)Tons of big and small command hooks that can take different weights.
3) Umbrella, rainboots.
4) Flocked hangers take less space than regular hangers
5) Rent a microwave/freezer combo from the college if available, or buy one. You can then stock a lot of things like popcorn and coffee etc.
6) Door mirror. Tool kit.
7) Shower caddy and a hair-product/styling tools caddy.
8) Iron (if your kid knows how to iron) and table top ironing board. A small sewing kit - needle, thread, safety pins and a small roll of "stitch witchery" or fabric glue.
9) Medicine kit - otc meds like Tylenol, Neosporin, Bandaids. and multivitamins.
and of course everything else your kid needs.