Mandatory questions since you left out pretty much everything:
Did you watch your brother sign up? If not, are you sure he ticked "No, create an account now." when entering his username and password?
What exactly did the website do after he clicked the button? "Still the same page" doesn't really explain much. Also, what username is he trying to register under?
Sanity checks out of the way, technical questions:
What browser was he running? Did he try and register more than once, and in a different browser?