If you didn't download the map packs yet, recover his account onto your box, download them on *your account*. Then redownload them on his (if it charges you for his account, forget that step, Sometimes I throw in a second step by accident) Have him recover his account and go to Recent Downloads and the map packs will be in there for free, that's how I got the Legendaries when they came out.
But if you have them already downloaded without the account there, Better get ready to buy some Microsoft Points.