Eaglefly when DS1 and DS2 were that size, I was buying XS man and XL boys sizes from H&M and (the now closed) BHS. Mandmdirect are really good for cheap, stylish, casual men's clothes sizes in size small (and medium and large too).
Unfortunately, I'm also in shopping mode, but it's for large size man, even though both are slim. Both boys take a 34in leg and their bodies are just too long for most medium size men tops. I spent yesterday bagging up barely worn clothes and donating them to a charity shop.
Annoyingly, I had to throw away all DS1's DofE clothes as none will fit DS2 when he does the expedition next Spring. They were only worn for two days and now I have to buy them all again.