How do you say are the only son in french?

There are a couple of ways to say "I am the only son" in French, depending on the context and formality:

Formal:

* Je suis le fils unique. (This is the most direct and formal translation.)

Informal:

* Je suis l'unique fils. (This is slightly more casual and emphasizes that you are the only one.)

* Je suis le seul fils. (This means "I am the only son," but it's a bit less common than the other options.)

You can also use the phrase "Je suis enfant unique" (I am an only child), which is a more general term and doesn't specify your gender.

The best option will depend on the situation and your relationship with the person you're talking to.