Ask your child if there is anything he needs, such as more socks or personal care supplies. Take note of any of his suggestions and purchase them if they are reasonable and useful things he will need.
Buy some guilty-pleasure snacks like candy bars or small bags of chips to include, along with healthy fare such as hearty canned soups, boxed pastas or anything that won't perish, but will be easy and delicious. If there is a favorite dessert or meal you make that can be freeze-dried and shipped, add this.
Buy him a few personal care items like toiletries and a package of clean T-shirts, underwear or some basic wardrobe pieces like a pair of new sneakers.
Purchase a small gift card from a favorite shop of his and let him have a mini shopping spree to ease some stress. This doesn't mean let him buy tons of new items, but $15 to $20 is a lot to a poor college kid and he'll appreciate the little bit of help and chance to get out of the house and go buy something he enjoys.
Take a few recent photographs around the house or at a recent family event he wasn't able to make it home for and include these. You could even place them in frames so that he can put them up in his room.
Write a letter giving him an update on how things are going back home and let him know you support him. Have any other members of the household also write a letter to show their support and show him that he is missed by all.
Purchase a box from your mail supply store large enough to accommodate all of the items. If you purchased clothing and it isn't pre-packaged, fold it and tie it together with a string or place socks in a plastic bag and tie it closed. Place any food items like candy bars or homemade cookies in sandwich bags and paper items like letters and photos into a large manila envelope.
Seal the box closed with packaging tape and write the mailing address in large letters using black permanent marker on one of the sides. Take it to your local shipping store or post office to find out the proper postage and ship it.