Hi toms 2850 and welcome to our UNOFFICIAL London 2012 Olympics forum. Well your son is certainly in the right trade because there will always be a demand for a decent plumber no matter where he decides to work whether in this country or abroad.
As for getting an apprenticeship at the London 2012 site in Stratford I would suggest he takes a look at the pages listed on the official London 2012 website which provide a great deal of information for people wanting to become involved in the project. I've had a look around and would suggest he views the Construction jobs and skills page at
http://www.london2012.com/get-involv...ills/index.php
Please bear in mind this is an UNOFFICIAL forum so can only provide you with general information for the London 2012 Olympic games. The official website address is
www.london2012.com
Please keep us posted on any progress especially if he succeeds in working on the 2012 Olympics project