Hoping for some advice from those in the know. My DS is 12 (nearly 13) and is 5 feet 9. Skinny legs but a round bum, so 12-13 will definitely be too small on the hips/bum. I usually buy for the season ahead of time but I'm in a hurry to get 1 slim/skinny pair (needed for school play) and don't usually buy that shape. I'm thinking of age 13-14 or 14. Anyone with a son with a similar frame found school trousers from the usual shops that fit better? Thanks. in advance.