London Paddington to Goring-and-Streatley station costs £16.40 offpeak (after 0927 weekdays) or £18 anytime. Direct services are half-hourly, there are also half-hourly services changing at Reading. Takes 1h09m direct. Cost of travel between LHR and Paddington extra.
Rather than going into central London, get the Heathrow Connect rail service and change at Hayes-and-Harlington to the First Great Western train to Oxford, for Goring-and-Streatley. Takes 1h09, costs £17.40. this is a NOT VIA LONDON ticket.
If you didn't notice from the wikpedia page, Streatley has a population of 974. For shopping etc then the 15-min train journey into Reading is probably useful, which is a major regional shopping centre, has two universities, and some major offices (Fujitsu, Hewlett-Packard, Microsoft, etc). There are also several local bus services.
If your son is under 26 then http://www.16-25railcard.co.uk/
There is also the Reading Music Festival in August. Apparently Guns and Roses are playing this year.
(I've hyphenated compound station names to make it clear it's one station)
Edited: 28 May 2010, 22:12