Most grocery stores are large and not all that intuitively organized. Placing staple items at the back of the store is more about getting you to make an impulse purchase on your way to the bread aisle than it is about making shopping easier. Within product categories, there is some organization, though. You'll typically find frosting in the baking aisle, and facial tissue will usually be close to the bathroom tissue. Start asking your child to identify the location of different items, and point out any store signs that can help him. Show him the call button for the butcher (if there is one), and alert him to the way like-items are displayed in the deli, the bakery and the refrigerated food cases.
Don't forget to let him in on a few tricks, too. You know that the freshest milk and other dairy items are likely in the back of the case, but he doesn't. Caution him to check that dozen eggs for cracked specimens, too. He's also probably looking for items along the center portions of the shelves at around eye level. Like many adults, he'll be ignoring the tops and bottoms of the shelves where good bargains are hiding. You don't have to explain that retail marketing can be a sneaky business, but do teach your child to be choosy and observant when he shops.