Guardian ship of Son

Dear Sir,

I am a 45 yr old Hindu, residing in Delhi and got remarried in 2006. My wife has a son (present age 16 yrs) from her first marraige. Her ex husband has nothing to do with her / her son and she has custody of the child. I too have a daughter from my previous marraige (age 14 yrs). My wife is keen that my son bears my name.
Three questions:-
(a) Can I legally adopt him or have a guardian ship deed executed in front of a court. Both my son and wife can depose in front of the magistrate. Her ex husband shall NOT give consent to such adoption.
(b) Can I be deemed as the Legal Guardian of the Child - for the purpose of applying a new passport for my son? Passport has space for fathers name / Legal Guardian.
(c) How do things change after the child becomes 18 - I am sure he can assume my surname? And can he then put my name as fathers name / legal guardian in legal / formal documents thereon.

My son states "If the Hindu Law allows a biological father to give up his son, so the son should also be allowed to give up his biological father"??
As you can note both my wife and son are very emotional about NOT wanting to have anything to do with the biological father. Would the fact that the said biological father did not provide for my wife and son post separation (05 years) despite a court order - help me to apply in an Indian Court??
Regards,
Anil