Sounds like you are sharing the same Apple ID for the iTunes Store.
If so, on your iPhone go to Settings > iTunes & App Stores > Automatic Downloads. Turn all off on your iPhone. If your son doesn't want to receive any of your downloads done direct with your iPhone, he needs to do the same on his iPod Touch.
iTunes on your computer includes the same but there is an option under the Apps tab for your iPhone sync preferences with iTunes to not automatically install new apps in iTunes on your iPhone when syncing.