I started giving my girls the shopping list and a pencil as soon as they could read (and actually, even before then, and would have them sound out the words). They're in charge of keeping track of what items we still need to find. I organize my list by the type of item (dairy, produce, deli, etc), so when we get to that section of the store, they have just a small list of items to look through. Then I'd have them help me decide which type to buy (maybe there's a 2-for-1 deal if I get the name brand that makes them cheaper than the store brand, or whatever). I ask, "If I buy 2, how much will it cost?" So that becomes a little mental math game.
Now that my girls are older, I will give them a piece of the list and tell them what their budget is, and we see who comes closest without going over - extra points if they can get more items than are on the list. (Say, if they can get one of those 2-for-1 deals!)
They also seem to like it more if they help me create the list in the first place. They go through the fridge and pantry and see what we're running out of, organize the list into the correct categories (we use Excel, so this is also familiarizing them with the software).
So, if you have the time to let him "help" you as you prepare to go to the store, or even just as you are walking through the store, he might enjoy that. And it will be a useful skill later in life, understanding how to organize a shopping list, stay within a budget, and even where things are located within the store (I wish my husband's mother had taught him this - I send him to get one item and it takes half an hour!).