"Mommy, my bird is standing up"

While driving in the car after dropping my daughter off for school this morning, my son and I were having a little conversation on our way to his school. His school is about 15 minutes away from her's so we can actually have a decent 4yr old convo.

All of a sudden, he takes a gasp of air and says "Mommy, my bird is standing up!" At this time I was driving and because he was seated directly behind me in his carseat, all I could respond back was "Oh, it is? Does it hurt?" I was laughing inside, but I did not want to show that it was funny so I kept my mommy face on. His response was "No mommy it doesn't hurt, I'm ok. BUT, when I move my hand, it's still standing!"

I was silent. Let's go back to what his first statement was... His "bird." He learned this word from his daddy. My daughter used to call it a tail but after she used the word excessively he made sure to change it to "bird" or "penis" as you know. So now, every time he feels the need to say something regarding his "penis", he says "bird." Now back to our little convo.

What do you say to a 4 year old who's just being introduced to his "bird" let alone his feelings while having his "bird?" I was not expecting this convo yet at all bc my daughter was easier at that she has no "bird" and this is my first time experiencing these 'boy' things. So I said to him "it's ok my love, it will happen sometimes but you will be ok." He smiled at me through the rear view mirror, said "ok mommy, Im ok now" and by the time that was over, we were at his school.

Whew, I thought about it and I think I did a pretty good job for a response. What do other Mother's with sons say, do or react when something as unexpected as this comes about?