Dogwatch,
Under Regulation EC 261/2004 passengers who are denied boarding are entitled to two things:
Care - meals/drinks/hotel accommodation in relation to the delay. If he has kept his receipts for any/all of the above he should claim for this. Even if he doesn't have receipts he should claim - for reasonable amounts. Some airlines try and avoid paying at place of departure, arguing that the passenger could simply go home - but this is not considered in the Regulation - your son should not be out of pocket for the airline's failing.
Compensation - For a 'long-haul' journey this would be euro600. There is no 'extra-ordinary circumstance' exception for over sales. They are required to pay, no ifs, no ands, no buts.
The airline should have first sought volunteers, i.e. people who were prepared to give up their seat. What the deal is between the passenger and the airline is not fixed - it's whatever they can agree between the two parties. If TCX did not seek volunteers (and it sounds like they didn't) they are in breach of the Regulation. In the UK the NEB (National Enforcement Body) is the
Airport Users Council. This is to whom a complaint should be submitted. While your son won't get anything out of this, the AUC (part of the CAA) does have the power to fine the airline for breaching the regulation. However, faced with a threat, it is more likely they will apply the Regulation in the future.
Good luck, and keep us posted on what happens.