You can't work on an oil rig without having completed the necessary safety courses (including fire-fighting and escaping from a ditched helicopter). The oil companies don't pay for these courses; potential employees must fund them out of their own pockets. So the first step for your son would be to to seek out relevant courses.
Relevant links:
http://careersadvice.direct.gov.uk/helpwithyou rcareer/jobprofiles/JobProfile?jobprofileid=10 99&jobprofilename=Offshore%20Roustabout&code=- 489041301
and
http://careersadvice.direct.gov.uk/helpwithyou rcareer/jobprofiles/JobProfile?jobprofileid=35 &jobprofilename=Offshore%20Drilling%20Worker&c ode=-1426021984
Chris



