Archana ji,
You have raised a really a BIG and GRAVE issue, a real 'grey matter', in subtle words
You wrote "We have been living separately now for around 5 years. I recently filed a divorce petition on grounds of cruelty. After a few dates, the judge asked us to go for mediation. In the mediation he basically said that he didn't want a divorce as he did not want to remarry. He doesn't care about the custody of our son or anything else. "
Females themeselves strikeout / discount their husbands as incapable to look after their kids in their absence ..... Then why would husband challenge that? 
.... In fact husbands happily agree to it!!!
Aren't you beating around the bush?
Real fact is you want to get rid of husband and also want to have a fresh start but for that you intend to shed cumbersome baggage if possible as well without hurting anyone's feelings, emotions as well as rights. It is a major factor if one wants a fresh start.
But you are a practical and strong woman who can boldly say that ................... 'if kids in broken marriages can stay, grow-up without a father's care, can't they grow without a mother's care? Aren't there any families where someone's mother gets expired after evn pregnancy?'
There is nothing wrong as well and a real sign of parity of genders in abilities, First thing is that one should not have a guilty feeling while thinking like this .... But one needs to take some more precautions .............
Why female are grappled with this emotional limbo that .............. if marriage breaks-up, automatically they are supposed to carry the kids along? ... But it is what practically happens!!
Or rather females separate out in such a manner that .... on this front .................. they inadvertantly go on closing the back doors!!
If females try to win gender sympathy, project a cruel picture of husband, and play badmouth against him, then how can they expect husband to come forward to take custody of kids? ... even courts would find it absurd!!!
One should be sensible enough to say that husband is a Good Father!!
Is this your agony that "he isn't fighting/comming forward over the custody of his son?"
Probably you want to get rid of marriage as well as any links to your husbands (ad hence also would agree to give custody of sone .... But husband isn't insisting on it!! Is that the dilemma? ) .................. It is a fact and stark reality that .......... remarriage options for women get narrower if one carries along the baggage of offspings from previous marriage and adoption issues often remain unresolved.
I am not making any bias against your practical thinking .... But The problem is with women that ................ whenever they separate out from husband over a fight ..................... they tend to drag their kids along .............................................. as if it is female's sole duty to care for kids ........................... and they themselves relinquish husbands from their burden/duty to look after their kids.
The alienation of husband from kids wins them sympathy in society but it gradually diminishes the chances for females to offer the custody of kids to fathers/husbands. Reason ? .. even kids start viewing father as cruel man and dislike to be with him.
IF females handle this issue with care and sensibly, females won't be blamed for abandoning kids as well as their husbands would also come to a realisation ................. what it takes to bring-up, look after a child!! ......
And it may turn out .................... as a surprising outcome ................. that both spouses understanding mutual expectations and problems of each other and probably reconcile.
