Suggestion:
If he has his own gamertag on the other console, then I'm pretty sure it's not possible. If he's playing under the same name, then you could just save his game to a memory card instead of the hard drive and then load that on the new one.
Suggestion:
If he has his own gamertag on the other console, then I'm pretty sure it's not possible. If he's playing under the same name, then you could just save his game to a memory card instead of the hard drive and then load that on the new one.