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.