I can provide you with a little help on what is available, what you decide to do is up to you. This support talks about Restrictions. That is the only type of parental control you can do within iOS. iOS: Understanding Restrictions (parental controls) - Apple Support
This talks about restrictions in iTunes. Use Parental Controls in iTunes to set content restrictions - Apple Support
Then you can take a look in the App Store for some other apps, mostly restrictive browsers to help filter content. They are not, nor is restrictions all inclusive. It will not prevent everything.
You are right that pay as you go with control some spending, it depends on how it reloads. Make sure it isn't automatic so that if he goes over, it automatically reloads. Again, most carriers have some type of restrictions a parent can put on an account, but I'm not sure if they are all available in pay as you go accounts.