This issue sounds fairly harmless; just regular adolescent curiosity. I think that as a concerned parent you can be aware, but not overly cautious. Just keep your eyes and ears open.

If the subject comes up, keep the discussion brief and use it as a "teachable moment".

The best way you can support your son is by modeling a positive attitude about body image and sex. You could share with him some of the doubts you experienced at his age. Probably the best bonding moment I had with my dad as during a long-distant car trip when I was 14. We openly talked about his first girlfriend. Even though I already knew i was gay, i could really appreciate what he was telling me. Also, I thought it was so cool when he pulled to the side of the road so we could take a piss together. That was TOTALLY out of character for my dad. I loved it!