It doesn't sound like anything to worry about. Curiosity is pretty normal, especially since he is moving along in puberty. I'm guessing he is kind of using you a a gauge to see what he has to look forward to when he gets older. If you are well endowed, that would probably draw his attention. There is a lot going on in the teenage mind and sometimes, its best not to pry. If your relationship is really open, he might talk to you about it but I wouldn't start up that conversation because chances are he wouldn't admit to it. Have you noticed him staring at anybody else or just you? The penis can be fascinating and while many might consider it rude to stare, its pretty natural.