I store all of my unopened cars in big Sterilite plastic containers. You can get containers that are stackable, which is nice for storage. I believe I got all my containers at Wal-Mart.  I bubble wrapped all the loose cars and put them in a plastic container as well. I would not store the cars in cardboard boxes because over time these tend to attract bugs. I would also not store the unopened cars  in a basement or an attic because these environments will most likely cause the the cards to develop an odor over time. The best environment for storage is the ground floor of your house. If you have to store them in a basement make sure you run a dehumidifier.  If you have to store them in an attic I would take them out if the temp is going to be really high – I have a lot of the Cars playsets that have stickers on them that could easily start curling if the temp is too high. I took pics of all my cars and made a list for insurance purposes, and so I can look at them once in a while without having to take them out of storage. This also helps me to keep track of what I have.  Every time I get a new car I take a pic and then it goes right in the bin. I don't have any advice for your son's cars  because all my cars are for collection purposes only – I obviously don't have kids yet.

Cherie said:

I would not store the cars in cardboard boxes because over time these tend to attract bugs. I would also not store the unopened cars  in a basement or an attic because these environments will most likely cause the the cards to develop an odor over time. 

I have NEVER had any storage items in cardboard boxes attract any kind of bugs, and the idea of mold smells, etc… in the cards really all depends on your home.  We are lucky to have a finished basement that is really the lower level of our home, so we have no issues.  I am also fortunate to have a hobby room in this lower level that I use to display my collection.  With only a handfull of exceptions, I am an opener and have my CARS collection displayed on shelves and in a display case I made from a pre-fab wardrobe.  That said, the walls of this room are full of my carded Matchbox and Hot Wheels collection.  Just like a 1:1 car collector that believes their cars are best enjoyed when driven as they were meant to be, I would rather display my collection rather than wrap them up in a box.  But…each person is different and that is what makes this world unique!!