Is there a way that i can set a limit for data for my son's phone. The current plan has 500mb and i don't want him going over this. I have looked on my account but can't find any option to do this and don't want any nasty surprises.
There isn't a way you can do it - at least not without your son's co-operation. There are apps which will give warnings as you approach your limit, or you can look at the usage online and manually turn turn mobile data off on the handset before the limit is reached.