Where to buy Clothes for teenage boys?

My son, (nearly 13) is thin… he’s healthy, but 8th percentile. 5’2 (today) and and 24 inch chest, 26 inch waist. 30 inch inside leg and growing. Nothing fits. Even my usual Slim fit shops, (H&M, river island) shirts are hanging off him. I’m doing okay for trousers (M&S super skinny slim fit school trousers) and shorts (H&M). T-shirts, ‘designer’ ones (sonetti, north face, McKenzie) all fit long and lean - and oddly, Tu T-shirts work as well - so he has ‘wardrobe’ T-shirts and drawer ones. :) However, I cannot find any semi formal shirts for holiday! Struggling with jeans too. (He doesn’t want M&S jeans!). Ideas, please.