Formal:
* Au revoir à votre fils. (This is the most literal translation and suitable for a formal situation.)
Informal:
* Salut à ton fils. (This is more informal and suitable for a friend or someone you know well.)
* Dis bonjour à ton fils de ma part. (This translates to "Say hello to your son from me" and is a common way to say goodbye to someone who has a child.)
You can also use a more specific phrase depending on the situation, such as:
* Bonne journée à votre fils. (Have a good day, your son.)
* Bonnes vacances à votre fils. (Have a good vacation, your son.)
* Prends soin de ton fils. (Take care of your son.)
Ultimately, the best translation will depend on your relationship with the person you are speaking to and the context of the situation.