You could buy goody bag but keep the account balance at £0 - that way he'll be able to use the minutes, texts and internet in the goody bag but not go over if he uses them all up - until you add more credit and buy another goody bag.
If you are using goodybags then they have a set data allowance on them, unless it's an unlimited data one.
As giffgaff is a PAYG network the account will not allow you to do anything that you don't already have on the account, you can not rack up bills for over using calls, data or texts. If your goodybag and credit has run out then you won't be able to use the service.
When a goodybag runs out of any allowance it will start to automatically use credit but if there is no credit on the account then you can not continue to use whatever has run out.
The only thing you need to make sure is that automatic credit top ups are switched off (they are by default). If this is on then the account will automatically top up the credit balance using the stored credit card once the credit has fallen below a set amount.
It's a payg network if u use goodybag when data is used up it will just stop connecting but if u have cash credit balance it will then use that till it runs out, also make sure that auto top up when less than £3 is turned off then it won't top up when u run low on cash credit from your bank