Paswan had announced Chirag's elevation a few days back and his formal appointment as the head of the key party body underlines his virtual number two position in the organisation ahead of his two uncles.
The 30-member Board has Paswan, a former Cabinet minister and Rajya Sabha MP, his two brothers - Pashupati Kumar Paras and Ramchandra Paswan, besides a number of other leaders, mostly from Bihar.
While Paras is the party's Bihar unit president, Ramchandra is Dalit Sena's head.
While announcing that Chirag would be the new head of the Board, Paswan had claimed that he took the decision under "popular demand" as the party's leaders and cadre wanted his son's elevation.