When your child starts to get curious about his genitals, is it appropriate to teach him the "correct term" (i.e. Penis)? Or to use euphemism such as "bird bird"?

I would not use nicknames or euphemisms. I'll be teaching my child the appropriate term as I don't see the need to hide the term behind a "cute" word like "bird bird" or "pet pet", whatever it is. Even children should be taught to acknowledge their body parts by its proper name. We don't give weird names to our hands and feet, so why use weird names on our private parts? Being embarrassed is not useful at all.