That's my boy

 

A father says, "That's my boy!" after his son scores a goal in a soccer match.

(Here, it expresses pride in the son's success.)

A coach says, "That's my boy!" when a team member follows instructions or performs well.

(Here, it conveys approval and encouragement.)

A friend might say, "That's my boy!" jokingly after a buddy does something impressive or amusing.
(Here, it's more informal and affectionate.)