We have the same problem round here, my dc are all tallish and slim. Next do great trousers for girls with skinny legs and the boys trousers also have the slimmest fit we have found anywhere. My dd's slim legs look beautiful, but I feel for my poor ds's. My dh still has really skinny legs and it is a nightmare to find jeans that don't look ridiculous!
In our house, the drainpipe style jeans have been a Godsend. No idea what I am going to do once this skinny jean craze goes out of style! In my experience, those "one size" jeans are more for the kids that are a bit bigger than they are for the skinny ones. Last time I checked, Gap didn't even stock skinny jeans for boys, but they do carry loads of husky.
Thanks, I have to say M&S were huge on him, this is the boy who wears "drain" jeans from hennes and they are still baggy on him! dh is super skinny too and wears skinny fit jeans (skinny and a habit left over from being a goth!) I will try Next!