Hi Kevin
Some good advice given here so far, but I would recommend going a different direction with your son's pre-war collecting.
Instead of buying a bunch of cards in low grades, why not save some money up and buy one nice card at a time.
An older gentleman once said to me "If all you buy is $10 cards, in 10 years all you are going to have a collection full of $10 cards". He meant no slight towards anyone and how or what they collect. He just knew me well enough to give me some sound advice and I understood what he meant.
Your son is 15 and in a few years he will most likely be in a better financial situation. When he gets to that point, he may want and will be able to afford pre-war cards in nicer condition. What will he do then with his lower grade cards? Probably sell/trade them off for nicer cards that he wants. There is nothing wrong with that of course.
I would skip that step though and pick up a few nicer pre-war cards now. If his collecting interests changes directions in the future, pre-war cards in better condition are easier to sell/trade.
Its nice to hear that your son appreciates the old cards and still enjoys modern cards too. On occasion I open a few packs of modern myself.
Hope this helps
Jantz