Tell him the medically correct name now and you won't have to explain a euphemism later. Also, later, s/he will know exactly what you mean by saying NO if a stranger wants to touch that.
There's a great joke here. When my baby was learning the words for his eyes, his nose, his ears, etc. He pointed and I told him "eyes" or "nose". It's one of those developmental things pediatricians put in their charts. One day we were diapering just after a bath and I ask him "Where are your ears?" He pointed. He put his hand on his eyes and said "Eyes", then "nose", then mouth" then "tummy", then he put his hand on his private part and I matter of factly, "That's your penis" He never pointed to it again, and we played the game every day after.
The pediatrician asked him the questions, for developmental charting. He went through naming everything, then pointed to it and proclaimed "That's my PENIS and YOU can't touch it!"
at 7:05 AM on Jun. 10, 2009