It is NOT an offence for a minor to own a RIF/IF, the law does not restrict ownership, the law restricts purchasing.
So as long as his dad is UKARA registered or has some other form of legitimate defence (We regularly go to our local store and are on good terms with the owner, he knows we are skirmishers and has no problem selling us RIF's even though our UKARA has not come through yet) he is allowed to give you a RIF.
BUT no money can exchange hands, yes we all know that we get our parents to buy us our first TT gun and that is how it is, the authorities know it and don't seem to mind, the airsoft community in general doesn't have a problem with you giving your parents money to buy then gun as long as it is two tone.
I for one however would be fully against a minor buying a RIF from a someone with a legit defence.