Basically I found at this age next to nothing fitted - because they can be quite tall but are usually skinny, so the proportions typical in adult men's sizing are all wrong
XS are next to never in the shops, so difficult to find anything to try on to see what the fit is like!
Tops are often easier - could be anywhere between XS/S/M
Trousers/jeans harder, but worth trying Uniqlo if you have a beanpole, as they do unusual sizes like 28/32 (ie small waist/long leg) which can be handy until they fill out a bit!
A few brands still do teen sizes - worth keeping an eye out (Asda used to be quite good, but I think it's a shadow of its former self, if the range still exists at all)