For what it's worth I think it might help to go for individual counselling to work out what you actually feel.
I realise that given the replies you're very defensive which is completely understandable but there are so many contradictions in your replies it's hard to work out what the situation really is.
My friend was in a very similar position. She had a son from a previous relationship and her new partner was hopeless and quite unpleasant to the boy. Long story short, against everyone's better judgment, they had a son together and with a lot of work and counselling as individuals rather than as a couple, and they're a very happy family of 4.
Being a step parent is hard and your partner must know that what's happening isn't great but perhaps you feeling defensive of your older son is not helping. They have to rebuild their relationship somehow, your partner needs to be reassured that you love your youngest as much as your first born and you won't play favourites and with respect, he may feel that you are doing that just now. I'm not saying you are, but you may be compensating a bit for his behaviour towards your son.
If he was a good step dad before then there is hope. There's bound to be an association for support of step parents that you should both look up as it's an adjustment that both of you need to make, there's always two sides to a problem.
Please get and accept support and advice, try and not be so defensive and hear what he's struggling with in terms of his feelings. The way he's feeling isn't uncommon but you both need to work to solve the problem individually and then together.
As I said if my friend and her family can do it, then so can you.