Refers specifically to the third-born son in a family. Often used in family registers, introductions, or when distinguishing birth order.
He is the third son, with two older brothers above him.
three sons
Used when counting or referring to a group of three male children. Less common than the 'third son' sense, and context usually makes the meaning clear.
He has three sons and one daughter.
He was born as the third son of a farming family.
The couple had three sons.