Who is treated as son ?

The son means Athmavai Putranaamaaseeth.

The self (Jeevaathma) is born in the

form of son what the sruthi  says.

The chaturvidha Purushardhas which are

Dharma, Ardha, Kaama and Moksha can

 be practiced only in the Gruhasthaasramam,

 but not in other Ashramas like Brahmacharyam,

 Vaanaprastham and Sanyasaasrama.

Through the Gruhasthasrama dharma man can  

only attain the self reliazation which is the ultimate goal of this life.

When he enters the Gruhasthasramam means

 for the sake of chaturvidha purushaardhas,

 he gets married to a deserved woman

 who comes  from a good family.

When once the gruhashta life  is  started,

he begets the progeny means son

through his Dharma Patni (married wife).

 So that son is called swaputrah.

If any boy who comes from his (gruhasthu’s)  swagotra

 family , that boy or that son is called as Khetragna Putrah.

As the gruhasthu does not have any children (son),

 when he takes adoption any boy from any

other couple of other family (which is deserved ),

that boy is called as Datta Putrah  (Adopted son).

If the gruhasthu  who has got  the child (son)

 through any other woman by whom she was

forcibly and frithtenly surrendered and gave birth

 to a boy, that son is called Goodhotpanna Putrah.

The boy who was born to a irresponsible

and haughtly person and left him alone

on the roadside, and that boy was brought

 from the roadside and bring him up by any

gruhastah , that boy is called as Apaddharma Putrah.

Before the marriage, if the boy is born to any

gruhasta , that boy is called as Kshetra Kumarh.

Like wise, the Dharma is said  regarding the son.