I feel your pain - I was lamenting exactly the same thing a couple of years ago (DS, now nearly 15, is now solidly into menswear sizing, thank goodness).
Gap, Next, M&S, Zara and supermarket kids sizes all seem to fizzle out at 12-13; added to which they are often very child-oriented in design (not many 13yo boys want Spiderman, primary colours or jaunty graphics).
H&M do at least go up to (I think) 14, and you can also buy online from Next in larger kids sizes (not in store).
Tops aren't so tricky, to be honest, as Mens size XS or S will often be OK. Or buy from Superdry women's range for hoodies. Hollister, Top Man, River Island etc. (all stores aimed at the younger end of the male market) are quite slim fitting too.
Trousers and shorts are really problematic, though. Primark was the one place that seemed to do really small waist size trousers and shorts in adult sizes (though trousers often needed shortening), and often there were loads in stock because larger sizes were presumably more popular.