Halfords will put him on the biggest bike possible it really depends what he will be doing on it he is probs a 15-16 anyway… I have never bought a bike from halfords but I know that they arnt great. Look along the lines of a hardtail, try and buy something with reasonable spec but don’t go for a budget apollo, he won’t have Mitch fun on that. Look for something like a carrera Vulcan or a specialized hardrock. There is a lot of choice out there for that price bracket. Also try and visit your local bike shop and not your nearest halfords branch. Most importantly never get your bike serviced by halfords their workshop is rubbish, go to somewhere like Evans!