How do you say but he will try his best in French?

There are a few ways to say "but he will try his best" in French, depending on the context and the level of formality:

Formal:

* Mais il fera de son mieux. (This is the most literal translation and is the most formal option.)

Informal:

* Mais il va essayer de faire de son mieux. (This is a slightly more relaxed version, but still polite.)

* Mais il va tout donner. (This means "he will give it his all" and is a very informal way of saying "he will try his best".)

* Mais il va se donner à fond. (This is similar to the previous option, but emphasizes effort.)

You can also use "cependant" instead of "mais" for a slightly more formal tone:

* Cependant, il fera de son mieux.

The best option for you will depend on the situation and the level of formality required.