Well, I'm 30, and I'm still reasonably infatuated with mine, so you might have another 26.5 years left to ride this out...

Having said that, it's a long time since I whipped it out in front of strangers...

I would try not to worry too much about it. It's annoying, but some boys develop different ideas about modesty than others, and it will pass. It doesn't sound to me like there's the slightest bit of inappropriate sexual knowledge or activity there, given his age, so just keep reminding him that you want him to play with it in private.

And I actually think you deserve commendation - I think it's great that he's so comfortable with his body.