You probably would want to start a new set just so he can get acquainted with multiboxing.

Personally I'd have him run a Paladin and a DPS class
While you Take a healer and 2 dps.

As long as one of you is controlling the tank and the other is directly controlling the healer you should be able to bring any DPS and be able to go far.

As a side note, do you plan on only levelling up together, or are you each going to level your group solo?