[andy]

One possibility is to have both on O2 UK. For the phone in France, O2's My Europe Extra add-on costs £10 a month and gives free incoming calls in Europe, 25p/min outgoing to Europe. The phone in the UK could be subscribed to O2's Your O2 numbers, which for £5 gives up to 1000 minutes to 10 favourite O2 numbers. For calls only between the 2, this is ideal, and the phone in France would dial a brief missed call and be called back by the other.

But of course, other calls will be made, and someone in France would be a bit anti-social having a UK number to tell their new French friends, so maybe the son would have 2 phones. On the other hand, from the UK O2 also has options for free international calls, conditional on a monthly top-up, e.g 100 free minutes with a £15 top-up, and the credit value can be spent on other calls

Of other UK SIM cards, Malkav has mentioned the virtual brands with cheap int'l calls; I think Orange's new Camel tariff will be cheapest at 10 pence a minute to French mobiles, though it's a bit more expensive for UK calls than their other options

For French SIMs, the ordinary tariffs aren't that hot, but there are interesting optional add-ons such as Orange's various unlimited packages, or SFR's 30 cents per call up to an hour, which would be good both for local calls and with a calling card or similar callthrough access to reach UK or USA