My 15 year old son refuses to wear a coat to school, however cold it is. He agreed he would wear a thermal top or vest but all the things I’ve tried are either too long to tuck comfortably into his trousers, or the neckline is too high and it shows under his polo shirt. He won’t wear a long sleeve thermal top as he wears a short sleeve polo to school. He is a very petite 15yr old, probably more of a size 13. Needs to be white. So far have tried Uniqlo, M&S, Next and Damart! Any ideas?