Bisquitine, if he is good at maths and going to get good grades, he should look at those Universities offering more mathematical economics courses, where there will be plenty of overlap between the two degrees. The most competitive are LSE, Cambridge, UCL and Warwick. All are very oversubscribed, as in they will have been turning down, depending on context, applicants with 4xA* predictions, and are very unlikely to be available now. Instead he should get his grades and spend the summer reading. Only Cambridge interview, so a strong PS is all-important.
Otherwise look a bit further down the Complete University Guide table (a tip is to rank by entry standards, as this indicates how strong other applicants are) at places like Bristol, Bath, Edinburgh, Nottingham and KCL and look to see who offers Maths and Economics or perhaps, in the case of Bristol, Econometrics. But you will still be looking at very oversubscribed courses. If he is A* material, he should want to be stretched. It is a very employable subject, but many employers will be looking for strong technical skills.