My old iPhone 4 was given to my son. He is 17 and has an existing contract with Virgin mobile. Obviously this is not a micro sim.

How do I swap him over to the new phone retaining his number if possible?

Ask Virgin for a new micro sim.

Ask Virgin for a new SIM, when it arrives call them and register it to his existing number, wait 10 minutes and it should be ready for use. At least that’s how it works with Vodafone, I’d imagine they’re all the same.

Take it to a phone shop and get the SIM cut down to micro. Or do it yourself with a Stanley blade.

Can you actually cut a large sim down to micro size? Or is this a wind up?

Sure. You can even buy a cutting device for less than a tenner (check eBay). Also look on YouTube for instructions on slicing with a blade. I did this when I went from an old Motorola to an iPhone 4. Not sure I’d like to try it for the nano-SIM in the 5, though.

google it you can get atemplate to print out to cut around

Done it twice by cutting a Tesco sim down to size for an iPhone 4. Worked both time. Normal kitchen sissors did the trick.
Get stuck in!

Just use scissors. Worked for me several times.