There are a couple of ways, you can do it with your computer, some software and a special cable.
you can pay some dude a fiver at the local market, or you can buy a thing called an unlocking clip from ebay. Clips on the bottom, unlock as many phones as you like.
Just contact Orange and ask them to unlock it. Providing your son has or had £30 or more worth of calling credit, there's no reason why they can't unlock it. You may have a 2 week wait though.