Oooh that might work too
I'll have to have a look into that for Carpe
You need an american phone number to activate it, but once google voice is activated it should work fine in UK/Ireland, and should be able to install it on android phones or use it from computer.
Also if he gets skype/vonage or one of the other voip services he can get an "american number". Meaning if his family in America dial a local American number it will be sent through the internet and will ring his phone, meaning his family will only be charged the price of a local call even if it goes to his mobile.
I would just get a smart phone and use skype or viber or any other similar voip app to call your parents. And skype out if they aren't too savvy. That way you can slap a uk sim card in the phone and local calls (to uk from uk) will be cheaper.
However, you will have to be careful, I know US carriers really try to lock down their phones and also some (verizon and tmobile, i believe) use CDMA as Fen said and those handsets won't work in the uk.
Skype is also getting built into the core of windows phone 8, so that could be a very good solution.
If his parents are not tech savvy you can get skype phones like this http://shop.skype.com/phones/cordless-router/ge-31
Wow thanx so much guys you lot have been a great help - now to just make sure he gets on the right flight