This is just my advice, but if your going to let your 6yr old son use your device, I suggest that you:
1. Change your password. Their is a chance that he might have memorized it.
2. Enable Restrictions and Turn Off In-App Purchases when you give your son your device.
3. Delete your credit that you have on file in iTunes and purchase Gift Cards instead. He will not be allowed to spend more than the Gift Card amount.
I have 2 nephews (ages 2 and 4) that I take care of frequently and they know my iPhone is off limits, but they are free to use the iPad whenever they want. I have downloaded several kid friendly apps that do not have Ads or off In-App Purchase (mostly paid apps, I read reviews prior to downloading) because I try to avoid this type of situation. I avoid apps with Ads because I don't want them to click a link that will take them out of the app and into the browser or the App Store.
My password is 12 characters long (mixed between letters, numbers and upper case) so that even if they see me type it in, its very likely that they won't remember it.
Is your device currently on iOS 4.3 or higher?
I ask because the new In-App Purchase policy went into affect with iOS 4.3. In the OP your claimed that your son was not prompted for your password while making those in-app purchases, so I'm just wondering what OS your on.
Apple changes purchase policy so your kids don't max out your credit card
http://www.tuaw.com/2011/03/10/apple-changes-purchase-policy-so-your-kids-dont-max-out-your-cr/