Can anyone recommend a good cheap source for international calls?
My sons travelling through South America, we call him daily and its mounting up.

skype is brilliant

skype is brilliant
embertech.co.uk

Thank you will have a look but are we able to ring him on his mobile using skype?

Yeah sure. I have skype for business and you can call mobiles at a lower rate than your landline phone, you add skype credits. Depending what phone he has, he might be able to download skype to his phone and install it, so you can call each other for free. I know that windows mobile phones you can. I have a HTC Touch pro 2 and connected to the free wifi in the hotel on holiday on the internet and called everyone using skype from my mobile phone so I didn't need to pay for any calls. I called landlines back home. It is VOIP technology (Voice Over IP) and millions of people are using it now.
Not sure what other options you might use being a mobile phone. You might get some other suggestions on this forum yet.

SKYPE is great. I used it when travelling and £10 would last over a month!
Other than that you could buy an international phone cards. My girlfriend calls home to Germany and this again lasts a long time on a £5 card. She buys hers from the local corner shop, it's called Talk Home Premium (the card not the shop).

I assume they have a local sim card for the country they are in.
Using a UK card will be expensive in the extreme.

Skype is very good.
Don't allow it to automatically top up as it will keep trying even after you don't need it any more. Check the call rate to the country in question and make sure that is the rate to a mobile.

Other options are access numbers. Google will give you links to several that you can dial direct from your BT phone. Many are very cheap and will just show up as part of your normal bill. You only pay the advertised rates and dial an 0845 number for the charging. With those be careful to check the conditions. Some are very cheap to land lines but expensive to mobiles especially in less developed countries. Some of the latter are so expensive they are on 09 numbers but the sites are usually very good and don't try to rip you off.

When I was making long international calls, I found calling cards also worked when using my mobile. I used a PAYG vodafone with stop the clock. That means I was paying 5p/min to call the London access number on the card for the first 3 mins and free for the next 57 minutes.
The cards usually cost 5 quid but the minutes you buy vary country to country and card to card. I found one called 'black' was very good for south east Asia.

If it's just a 'check in', SMS is also handy.

Added. Most countries seem to have internet access from mobile phones. That means he can upload photos from his phone to such as photobucket.
Handy to get an idea of what he is seeing out there.

Bugger, forgot.
These cards are usually available from corner shops run by foreigners in area where there are lots of other foreigners. It's a common way for them to call family from their home countries. Many are geared up for calls to the Indian sub continent to be careful what you buy.
Internet searches will give you an idea.
Try 'black' first.
The shops usually have charts up stating the call costs.

FREECALL, recommended a long time ago by the moderator of this tec forum, works great and it never lets me down, google freecall.