Adoption of Widow Daughter's Son in West Bengal

A hindu person, not having a son, adopt his own widow daughter's minor son. Adoption deed registered at sub-register office in West Bengal. But after his death, unfortunately, the Railway official refused the compassionate appointment of the same adopted son by saying that the 'Adoption Deed' is invalid in the eye of law. Now here is my query regarding this. 1) Can a Hindu person adopt his own widow daughter's minor son in the eye of law? 2) How could the adopted son proof that the 'Adoption Deed' is valid in West Bengal?