In Carrie's War she was given an apple, a ham sandwich with brown bread and a bottle of water. I would think something like a jam sandwich or a meat paste (if you son likes it), maybe a piece of fruit cake and either water or milk to drink.
Hallo Julieboo, Dont worry about it,I was at school then,and with little food to choose from,lived on bread and jam sandwiches and at the age of 16, took up Rowing as a sport, which is as hard as you can get, so it did me no harm
My Mum and Auntie were at school during the war and didn't take food to school - they went home for lunch (well, actually dinner as it was in the north!).
It wouldn't be the nice white sliced bread we get today. Probably brown bread or wholemeal (Hovis would be authentic, but not pre-sliced), and you wouldn't be able to wrap it clingfilm or tupperware either. Waxed bread wrappings or greaseproof paper would be about right.
Apples would have to be English - perhaps a Cox or a Worcester. Certainly no bananas.
Of course, you could send him with his entire week's sweet ration. I think it was about 2oz, but maybe an older ABer will put me right there. Dolly mixtures or jelly babies so you get more for your ration.