Water, cordial, maybe a lucozade sport but I think water is fine at that age. My sons 14 and just been signed by a professional club 15 months ago. Make sure he always fuels his body, mine has always put that extra in durning games and training, it’s a f***ing nightmare now though him wanting all his special meals beofre and after training. Eats a lot of pasta and 5 weetabix with blueberry’s and raspberry’s a couple of hours before a game on a Sunday. Then will eat a banana on the way an hour beofre kick off. He also drinks a lot of water through the day. I’d say he drinks atkest 3 litres a da