Boy wants a bra

Picking up my bra from where it sat on the toilet, my son asked with a smile, "Do you have no more milk?"
I continued drying off from my shower. "Nope. No more milk."
This prompted him to come out with yet another utterance of his new favorite word: "Why?"
"Because you're growing up."
"I want you to have a baby so you can make more milk," he suggested.
This is the second or third time he's said I should have a baby. Don't jump the gun, little boy. We're not quite there yet... As a reminder, he said with a glint in his eye and a giggle in his throat, "Can I nurse?" I returned the laugh in my "No, you don't nurse anymore," given with a hug.
Then he asked, "Do I not have breast-es?"
"Just small ones."
"Why?"
I generally feel like he doesn't need a scientific explanation for anything, but just saying "hmm..." or "I don't know" or "what do you think?" doesn't always satisfy. If I answer something right away then I avoid a continual string of "whys." Sometimes I just say "because you don't." This time I explained, "Well, you're just a little boy."
He countered that he was a big boy. Then he held the bra up to himself and asked, "Can I not wear this because I just have little breast-es?"
What response could I possibly offer, besides a laugh?