iPhone for England and US?

The iPhone is sold in the U.S. as carrier locked with AT&T only. The iPhone is sold in the UK as carrier locked with O2 only.

International roaming is very expensive. O2 also offers a pay as you go option instead of a contract, so he would be better off in the long run (less expensive) by purchasing an iPhone in the UK under the pay as you go option. He could sell the UK iPhone upon his return.

An iphone purchased in the U.S. cannot be activated on another carrier, it is carrier locked to AT&T and cannot be activated with any other carrier.

If he breaks his two year agreement in the U.S., he will have to pay the early termination fee. If he wants an iphone in England, he will have to buy one in England. It too will be carrier locked, but to the carrier in England and cannot be activated with AT&T when he gets home.

The financially smart thing to do is to put his current ATT plan on hold. I don't know if it's possible with ATT or your family plan but it's possible for a few companies in Canada. You only pay a few dollars a month to keep you number, and then resume your full contract once you get back, it's often called a "vacation plan" or similar. Once you son is in the UK, he can then purchase a very cheap phone and perhaps a pay-as-you-go plan, some PAYG plans offering unlimited incoming, which is nice because then you can call him without having him using heavy minutes.

If he really wants to have an iphone in the UK, it's gonna cost an arm and a leg. You can sell his phone in the US and if you can't put the plan on hold, you should pay the early cancellation fee or simply pay off the contract depending on how many more months is left. Then he can purchase a legally unlocked phone from Hong Kong or Australia (apple sells them there, and they are quite expensive) and add O2's PAYG plan once in the UK. Once back in the US, he can either get another iphone plan again or switch to another plan or company (t-mobile).