My son is 10, of fairly average height (roughly 145 cm I believe) but is very slim. As such, to get clothes that are long enough in the leg or the arm, they then swamp him. We have one absolutely fantastic pair of slim fit trousers from Crew, which I have bought in a few sizes to last him, plus some slim fit Boden jeans, so trousers aren't too bad, though choice is limited.
On the top half though, it's a nightmare. One or two of his 8-9 items from last year still fit his top half but won't for much longer. The next size up from most makes, 10-11 just are enormous, past the point of being fashionably a bit baggy. We have Gap, John Lewis, Crew, Fat Face type clothes as a rule which he really likes the style of. He needed to look smart today for somewhere he was going with my husband and we tried on a few things which all look like they won't fit him until next year, let alone this one! Luckily, last year's smart jumper did the job but he can't wear that every day.
If your son is slim up top, where do you get tops/jumpers that fit the body but are long enough in the arm and broad enough in the shoulders?