issues with surname

[tycouk]

Hello all,
I am going to apply visa for my son(step-son) from India,we have recently granted his legal guardianship from court.in his passport the surname is left as blank, from his birth he has been living with her mother,so never mentioned father's name any where in his schooling,but they mentioned biological father name in passport. we could not able to get my name on his passport as legal guardian . so could you please some one suggest me how would i fill the application.your help is much appreciated. I have asked some questions before in this forum but not this question.
please senior members suggest me, as I am going to fill the application very soon.
Thanks