Topogigio's answer is correct but incomplete.
To make an international call (from anywhere) to the UK,
you first need to dial the international access code which the country you're in uses. (The code varies between countries. The one used in Canada is
011). Then you dial the code for the UK (which is 44), followed by the UK number, omitting the leading zero.
So the number your son needs to dial begins
011 44 1685
That's confirmed here:
http://www.thephonebook.bt.com/publisha.conten t/en/search/international/record.publisha?id=L 40
That will work from both landlines and mobiles. However, mobile phones normally have a + key. (On my Nokia phone, it's accessed by pressing the * key twice). That's a very handy facility because, whatever country you're in, it replaces the international access code. So your son can use this number, on his mobile, from anywhere in the world:
+44 1685
Chris