What is he spending that amount of credit on?! Orange have unlimited texts if you use £7.50 of your credit to buy a bundle that lasts a month. Something like that might be an idea. Get him to email all of his friends or Skype them if he has a tablet or smartphone with wifi and disable the mobile Internet on his phone. That's what I've done with dd. .
£15 month! I don't spend that on my phone and currently I top up the dcs the equivalent of £5 a month which, dd manages on very nicely by buying a text bundle, using her free credit and only surfing via our home broadband.
I think maybe I have to give up any idea of ds keeping in touch me and his dad via text, I don't have (or want) a smartphone so email and so on not an option.
It's data that ate all his credit btw afaik most of which was used at home in his bedroom, within range of the wireless.
I have a sim only deal with Tesco for £7.50 per month, it gives me unlimited texts, calls and 500mb data. I think I signed up for a 12 month contract with it about 3 years ago. Might a contract like that be any good for you? He could pay the extra, over and above what you would normally want to pay for his top ups (my 14yo dd has this contract too and she pays £2.50 per month towards it, out of her pocket money)