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 suggest that you use both. Teach him the proper term and also the euphemisms (e.g., some people will refer to it as “bird bird”). At the same time, introduce to him the concept that his private parts are areas that should be kept “private”. Use this as a time to teach him about molestation. Explained that his private part should not be touched or shown to anyone except mommy or daddy during a bath or dressing, or to a doctor during a checkup (with mummy or daddy around). Educate him to inform you if anyone touches him inappropriately.

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.

Anatomical names only for me. By not using the correct terms it makes a child shameful of their body parts when infact it is no different from their hands, fingers, toes, etc. The American Academy of Pediatrics states "It is important to teach your child the proper names for body parts. Making up names for body parts may give the idea that there is something bad about the proper name."

I advocate for using proper names for body parts. It demonstrates ownership and pride in our bodies. In my experience, I find that when parents trust their children to understand certain concepts after an honest explanation, kids can surprise them with a logical response. At least, one could hope.