Oyster cards aren't valid on that National Rail route. The only way that an Oyster card could be used to travel from Wimbledon would be to use the District Line underground service. (32 minutes direct to Embankment. Then walk over Waterloo Bridge - or take a Northern Line/Bakerloo train to Waterloo, but it's hardly worth the effort. �2.70 is deducted from the card for each journey during peak hours (0630-0930 & 1600-1900) or �2.20 off peak).
Student railcards are subject to a minimum fare for weekday travel before 1000 (which is far more than the standard fare).
An Anytime Return ticket, on the National Rail service from Wimbledon to Waterloo, costs �5.30. If your son will only be travelling to a from work, for 5 days per week, that would be his best option. However if he'll also be travelling around London at weekends and/or in the evenings, a 7 Day (Zones 1 to 3) Travelcard might work out cheaper, at �30.20.
Chris