Buy him the next season for battle royale.The new season should start in two or three weeks and cost about $10.It's not a pay to win game,so dlc is purely cosmetic.Skins and guilders alone can cost ten to twenty dollars.Pickaxes are usually around eight-fifteen dollar range.
If you buy him the forth season he'll get like three,four skins and around two,three guilders and picks.Not to mention lots of other cool cosmetics,like contrails while skydiving and dances.
It cost 950 in game currency called v bucks, and if your son can finish the season he'll unlock 1,500 v bucks along the way.
Plus he'll most likely love unlocking everything.
Keep a eye on Fortnite twitter or just get your son to let you know when the new season starts.