It's probably legal, if rather unethical.
In general, anyone who is in the employment of another person must receive the national minimum wage. There are only a few exceptions but one of these is 'voluntary workers'. The exemption was written into the law for perfectly good reasons. (For example, many charity shops would have to close if they suddenly had to pay all of their voluntary staff). Unfortunately, certain unscrupulous employers seek to take advantage of this 'loophole'.
http://www.direct.gov.uk/Employment/Employees/ Pay/PayArticles/fs/en?CONTENT_ID=10027201&chk= 00/jcP
If your son sees the job as a stepping stone to something higher, with a better employer, it might be worth the sacrifice. However, if the job only offers a 'dead end', where he'll be stuck with the same lousy employer for years, he'd probably be best advised to reject the offer.
Chris
PS: HMRC are responsible for ensuring that firms pay the national minimum wage. Irrespective of whether your son takes up the offer, it might be worth phoning their hotline to let them know about how this employer seeks to get around the legislation. (Details are in the link above).