I understand that when they are very young, the foreskin doesn't actually peel back properly anyway, it's still attached to the glans, and you shouldn't force it or you might damage it. Since no-one should be sniffing under a five-year-old's foreskin anyway, that isn't usually a problem.

I would say that you bring it up during the traditional "birds and bees (and whatever elses)" talk. Simply point out that a potential partner appreciates a pristine pecker, and that should they some time in the future intend to have sex, they should be thoroughly cleaned before hand. Although not with a steel-wool scourer.

Hell, just tell them to do it whenever they shower. Easy. Really, there's no need to "handle the uncut thing" - just tell him it's a part of the body that needs cleaning, just like every other part.