Add a phone to my iTunes account?

You will run into trouble by doing what you suggest.

The ability to get his data back from your iCloud account to his phone on his original account will be tricky.

All sorts of sign-out and sign-in will lose lots of settings, wallet etc.

He would be better backing up to his or your computer, or to his own iCloud account.

How does he back up at the moment?

Chaging SIM cards does not entail any loss of data. SIM cards do not store data,

This is all that needs to be done

Set up imessage and facetime after change of SIM/number. 

Settings, Messages, Turn off iMessage

Settings, Face Time, Turn off Face Time

Swap SIMs

Restart the phone.

Settings, Messages, Turn on iMessage, SMS, MMS

Settings, Face Time, Turn on Face Time

You can add another device (with the same Apple ID) to your account, see -> Check your Apple ID device list to see where you're signed in

Check your Apple ID device list to see where you're signed in - Apple Support

FWIW, As the OP rfefers to another person's iPhone it will (should) have a different Apple ID and the OP's suggestion is unworkable.