The name he chose is already taken.
Tell him to pick another username.
TIPS:

# In general, the more unique the username, the more likely you'll be able to use it for various websites, and the less you'll have to remember. On the other hand, if you make it too specific with personal information, your safety and privacy may become an issue.
# Use a pun or a variation on a common phrase or title. When in doubt, mock the name of a popular song, movie, restaurant, or store.
# Using numbers is usually a last resort, and are considered by many to be tacky and uncreative. Plus, they're not the easiest things for others to remember you by.
# Using underlines or dashes can be useful, but often hard for others to remember if they want to type your name into a search bar.
# Come up with a list of adjectives that describe you and find a way to incorporate them into your username.
# Don't spend too much time on deciding your name. A good name is one that you should be able to remember. Don't make it so complicated that you will forget it.
# Use "Search"function/QL for the name if you want to find out whether it is unique or if someone else used it before on QL.