If you are governed by Hindu Law, the Hindu Adoptions and Maintenance Act (HAMA), 1956 strictly dictates that a person cannot legally adopt a son if they already have a living biological or adopted son, grandson, or great-son. Since you already have a biological son from your first marriage, an adoption of a male child under HAMA would be legally invalid (void).
To bypass this restriction, you and your wife can choose to adopt under the Juvenile Justice (Care and Protection of Children) Act, 2015 through CARA.
Once a child is legally and validly adopted (under the JJ Act), they are considered your legitimate child for all legal intents and purposes from the date of adoption, it does not adversely affect or reduce the pension amount, but it changes the line of succession for the pension.Children: Children only receive the family pension after the death or disqualification of the spouse.Defence family pension rules stipulate that pension is paid to eligible children one by one, strictly in the order of their birth. It is payable to sons until they turn 25 years of age, get married, or start earning a livelihood (whichever is earliest).