There's a lot of stuff for someone starting out in the hobby. Most of the links below are Amazon but I strongly recommend you make any purchases at the local gaming store if that's an option. Here are some ideas:

-Players Handbook, everyone should have one. It all the rules to play the game and make characters
-Dice, everyone needs them. Buy some in his favorite color(s) or even fancier metal dice
-Starter Set, it has everything a new player needs to run their own games. Includes dice, an adventure, premade characters
-Dry erase battle maps to map out where the characters are exploring
-Miniatures are helpful. You can paint your own or buy prepainted