Can baby boys already inflate their *****?

The anwer is no, but this is what happens when you leave me to take care of a baby...
It all started out when I was playing with my little brother and he was climbing on my arm, while doing so I felt something just explode and grow.

My expression was like "WOW! My little brother can inflate this penis! " Then I had another thought that he had just pee'd, so I changed his diaper and found I was correct. He did pee.