由英國打電話返香港

你可以叫你兒子去到當地買一張sim card(地點如vodafone，orange，o2，etc)，然後top up(增值)，你可以在香港打電話比佢，佢接聽電話係唔使錢的~ 係個比較便宜的方法!
佢亦可以打返香港，之前orange有6p/min的plan(唔知而加仲有無)~
另外，佢可以另外再買一張國際電話卡(在china town通常都有)，用手提電話打晒卡上指定的號碼+hk的number，就可以打返來~
但呢d電話卡，有d會charge電話card+手提果張儲值卡，有d就淨係charge電話卡~
最方便當然係佢去到當地即刻買張儲值卡，比個number你地，你地定時打比佢，就最慳錢~

你個仔幾大先？要落唐人街，要買Pay-as-you-go SIM識唔識先？

buy a SIM card over there, in uk, pick up call doesn't have to pay. (Cheapest, but only one way)
So you can call him.
If you want him can make call to you, ask him to buy phone card over there. dial the phone care number + PIN. It would charge twice, the local call fee and the phone card charge the overseas call.