If it's the app you download to be able to shop the online Apple Store, the one that sells Macs and such, your son may have removed it, in which case you'll need to download it again or sync it from your computer.
If you mean the App Store or iTunes apps, those cannot be removed, so as KP said your son may have move them to another screen, set up restrictions to hide them, or placed them in a folder (look for an icon that looks like a black square with small icons within it).