I have a similar problem. Ds2 is a different shape to ds1, so whilst the skinny fit suits ds1, I have to find other trousers for ds2 to wear. I've found H&M are quite good, they do their jeans in different fits. Also Next are quite good.
My dses are younger though, so hope its the same for your ds!
The biggest problem is the current fashion. Everything seems to be either skinny or slim. He's just not the right shape. He also has this incredibly high instep which makes it impossible to get his foot/ankle through the foothole of narrow-ankled trousers. Boden just did really sensible straight-leg / baggies and I need to find a substitute. We traipse from shop to shop, not finding things, then trying things on but they're awful and it's really soul destroying for all concerned. We tried Gap the last time but they were hopeless - it's probably worth going back as they change all the time. Any more experiences much appreciated!
Gap? Husky fit? MY son was exactly the same, M and S plus fit school trousers and Gap husky jeans were my saviour. But about six months ago he shot up tall and everything is now hanging off him! Bhs plus fit stuff ( I think they call it sturdy) is ridiculously big. NExt plus fit also good, I bought two pairs of jeans a while back which were spot on, but now too short and falling down!
My son was like this - in the end I used to buy adult trousers from BHS - they had half elasticated waists and were roomier than standard trousers. I think the trick is to go for small adult sizes instead of trying to find large child sizes.
Does anyone know where I can get trousers for a big 11yo? He is tall, fits 13-14 tops now, but cannot wear skinny trousers or anything slimfitting. Until now, Boden has been the only brand that has been any good, as long as we get a straight style, and he is currently in a 13, but they have just changed their top age to 12. We can generally find jeans in M&S but nothing else. There must be other people out there who have kids who are not the right shape for skinny trousers - what do you do?!