"Il faisait du sport, alors il était très costaud."

Translation: He used to exercise, so he was very strong.

[Shelbyc134]

Is there a difference between costaud and fort or a certain time you should use one over the other?

[Jojo553168]

Fort has many meanings and is very versatile. It can mean, depending on the context, strong, clever or smart, gifted, or big.
Costaud is mostly used for muscular strenght, and is a bit less formal..

[Stacysdad]

That makes sense, thank you!