The DnD starter set is excellent, and has everything that's necessary to play (except for people to play with) If you are looking to do more than that, the Dungeon Master's Screen reincarnated is a good option, if you think he will be the Dungeon Master. Make sure the starter set and screen are for 5th edition.
Another possible addition is more dice. The starter set comes with one set, but it's handy to have a set per player. Wiz dice and Chessex both have interesting colors. You want either individual seven dice sets (20 sided, 12 sided, two 10 sided, 8 sided, 6 sided, 4 sided; the two 10 sided dice are sometimes called percentile dice) or a collection of several sets, sometimes called a pound of dice. Wiz Dice offers some collections in a large bag. There's also the Cup of Wonder which has five sets of dice and a dice cup.