How do you say this is my son in french?

There are a few ways to say "This is my son" in French, depending on the context and formality:

Formal:

* C'est mon fils. (This is my son.) This is the most common and neutral way to say it.

Informal:

* Voici mon fils. (Here is my son.) This is a bit more casual and implies you are introducing your son to someone.

Other options:

* Il s'agit de mon fils. (This is my son.) This is a more formal and less common way to say it.

* C'est lui, mon fils. (It's him, my son.) This is a more poetic way to say it, and implies you are proud of your son.

You can choose the most appropriate phrase based on the situation and your relationship with the person you are talking to.