There are quite a few misconceptions on this thread.
A non competition covenant is, in theory, fully enforceable if it is concisely drafted and goes no further than is reasonably necessary to assist in protecting the employers legitimate business interests. That applies even if it deprives the employee of employment in their chosen field and it isn't deemed a breach of human rights.
Salary and seniority are however taken into account when considering if the covenant is reasonable.
However, it's impossible to say whether this particular clause is enforceable or not. Firstly, I would need to see the wording to check for any drafting errors. Secondly, the fact that he is paid minimum wage and working in retail points away from it being enforceable - but the fact that it's a specialist area points towards it being reasonable. Thirdly, the covenant may well be properly drafted as six months is a reasonable time restriction, it has a geographical scope (although you don't say what that is) and only relates to relevant business activities which again points towards it being enforceable.
Basically a non compete covenant is usually designed to prevent the employee from passing on or using specialist knowledge - such as clients, prices, suppliers etc. If it's a narrow field and a lot of damage could be done by an employee working for a competitor then it could well be valid even though it's a restraint of trade.
He would have to do the ex employer a lot of financial damage for them to want to spend circa £10k in legal costs on a high court injunction, though...