There is nothing we can do for you. The only thing your brother could try, is to send an email to Hotmail Support, giving as much details as possible, to show his account has been taken over.
If that person comes online with your brother's address, he could impersonate your brother and make it look like your brother says/does/thinks certain things, while he doesn't. That could be annoying.
You could try
http://support.microsoft.com/default.aspx?pr=cntactms and
http://www.hotmail.msn.com/cgi-bin/support for support from Microsoft regarding this.
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