Well, if the doctor has put you on Anti depressants, then he obviously thinks you have PND< so I would go back to him. Research has shown that anti depressants work best when combined with other therapies, such as counselling.
On the other hand, maybe you just did so well with your first child, that you expected this one to be the same, and when things became difficult you felt disappointed an that feeling has grown.
I really doubt you don't love your son, even if you feel like you do. I had PND with my first son, and when he wa ssix week sold took him to my Mum's for the night so I could go out, and didn't want to pick him up again. I left him there for a week before my Mum demanded I collect him. I felt like he was not my son, but just this baby I had to look after, like he could have been anyone's baby. I think I had heard so much about 'bonding' and how wonderful women feel when they have their babies, and I just didn't feel like that. I had a dreadful labour, and I remember counting his fingers and toes when they put him on me after he was born. I didn't even carenif he had them all, I just thought that was what you did.
I didn't really feel maternal until I had DS3, and even now, pregnant again, I realise I am just not 'that type', not at all Mumsy, but I do love my children, deeply.
Go back to your GP and explain how you feel. Anti depressants can take some time to really start working, but not all GP's are that great at dealing with depresson.