You don't need a law A level, in fact some Universities have quite a strong preference for you not to have it - because the way you learn law is rather different at degree level.
Only having two A levels will certainly be an issue at some of the top universities, but he may be able to get in.
If there's a reason why he's only doing two, such as having to support himself through substantial paid work or having family responsibilities, then he should highlight this very strongly in his personal statement - he MAY just be able to get in with less than the usual requirements if that's the case.
Economics tends to be on the preferred A level list for those Unis that have them, while sociology is 'neutral' (and something like travel and tourism would be 'poor')