Why a mother needs NOC from biological father?

When I divorced my ex-husband in the year 2010, I was declared the soul custodian of my son. Since then, I am looking after my son -- physically, financially, mentally and emotionally. I am responsible for my sons wellbeing, health, education among all other responsibilities parents are supposed to take of.

In so many years, my ex-husband never wanted to know about our son. Nor did he take any responsibility including financial and emotional.

After 7 years, I remarried in the year 2017. Now, me and my new partner are together looking after my son. Now my present father wants to be the legal guardian of my son, he has to officially adopt my son, right! We are ok with all other procedures except ONE.

* Now the fight starts. For the adoption procedure, we need an 'NOC' from the biological father.
* Why a father who is grooming the son needs a 'no objection certificate' from the father who refused did not take any responsibility?
* I am the mother. Why do I need to ask the biological father while taking any decisions.
* It is my right to take all necessary decisions for my minor son, unless ex-partner had shared the responsibility.
* In my divorce deed, it was clearly mentioned that ex-husband had willing given all rights to me. Then why do I need to ask him again?

I want my right to decide for my son be preserved. What if I deny to ask for an NOC?