I think it depends on how much you get. If your X in on centrelink payments, CS is only about $25 or something a month, so I guess it wouldn't cover much. A pack of nappies?
Some people get alot more though & because when CS goes up your centrelink payments go down, then it would go to other things like food & rent.
I think its your choice where it goes. But thats only the amount he's entitled to pay. He doesn't legally have to pay any extra at all. Even for activities.