The most common ruck sack used in the Q course is the issued MOLLE pack. He will be fine using that... Some guys use an ALICE pack (large) and that can be purchased online or at most army surplus stores. If you are trying to go high end some guys like tactical tailor MALICE packs.
http://www.tacticaltailor.com/malicepack.aspx
Most guys at group have multiple rucks depending on the mission/training event. (MOLLE, ALICE, Mystery Ranch, etc) But they will issue those to him, no need to buy anything.
I agree with the above comments... ask him what he wants. Keep in mind he might need some more time rucking before he knows what is comfortable for him.