If all your son wants is a gap-year experience, an online TEFL course will suffice. It doesn't really matter which one as online courses are much of muchness. Be advised though that an ever-increasing number of employers expressly state that they won't accept them. This is because they don't include the all-important teaching practice, e.g., where you teach real students as opposed to your fellow trainees.
If he is interested in a possible career (or he wants to teach in a native English-speaking country), he will need to have something a little more substantial. This means taking a four- or five-week intensive course, with a minimum of 6 hours' observed and graded teaching practice. The big hitters here are the Cambridge CELTA (http://www.cambridgeesol-centres.org/centres/teaching/index.do
), Trinity CertTESOL (http://www.trinitycollege.co.uk/site/?id=201
) and, though newer to the market and thus well-known, the EDI CertTEFL (http://www.ediplc.com/certtefl.asp
). They are all accredited at Level 5 on the National Qualifications Framework and are as internationally-recognised as it is possible to be.
FWIW I took an online TEFL course as a sort of a 'taster', after which, having ascertained that TEFL was for me, I did my CELTA in Spain (you can find course providers using the link I gave above). That was three years ago and since then I have taught in Vietnam, Portugal, Poland, the UK and Spain.
Hope that helps, and if you have any other questions please ask.