That aren't jeans, chinos, tracksuits or formal wear. Is that too much to ask?
He needs trousers for running about in. I don't like him in tracksuits except for sport and in any case he's tall and skinny and needs a belt/adjustable waist. Jeans are OK, but not good if he's out all day in the rain (with Scouts for example). Previously he's worn combat/cargo type styles, but I can't find anything atm.
What do your skinny DSs wear to play in?