Have you considered math competitions? Even if DS doesn't want to compete, you may be able to find out about math enrichment opportunities in the UK. Try contacting someone at the UK Mathematics Trust: http://www.mathcomp.leeds.ac.uk/about-us/.

Another option is Art of Problem Solving. In addition to courses, they have communities that might keep DS active in interesting math until he's ready for University: http://www.artofproblemsolving.com/