I was thinking about this thread last night. People have said how would the child know that his SM doesn't like him.
What came to mind, is that if someone can have such resentment to the degree they wished you didn't exist and act as though they like you to your face (and in this case put on the same act in front of his dad), it's really quite disturbing.
It's also a potentially dangerous situation for both you and your SS.
There's a world of difference between not liking or not loving someone, to resenting their very existence.
I actually get the point that your view on his DM can affect your feelings towards him.
I remember feeling the same about a woman in our community (I was mid teens at the time) who had an affair with the DH of another woman in the community and she had a baby from the affair.
I just didn't want the boy anywhere near me when we were all at an event together. Where people would normally coo over a baby or want to carry him, most people stayed a distance away from him.
He had done nothing wrong. He was an innocent child,
just born into a mess that wasn't of his making, but it didn't change how people felt about him. It was his DM they didn't like and what she did.... with no guilt or remorse.
A lot of the women in the community didn't want their kids to even play with him, which I'm sure he felt as he grew up. If he asked for a drink at a party, he generally got ignored (people acted like they didn't hear him) or told him to go and ask his mum. It would otherwise be the norm in the community to get a drink for any child who asked you at an event.
It didn't help that as he grew up he was a naughty kind of kid.
So while the saying is not to pass the sins of the father (or mother in both cases) onto the son, human nature isn't always controllable.
The difference for me, is that I didn't see the boy on a regular basis and didn't have to do anything for him.
So while I understand how his DMs previous behaviour has more likely than not influenced how you feel about him, perhaps focus on the fact that 50% of him is made up of the man you love and that he shares that 50% with your DS, his half brother. Think about how it would feel to see your DH in pieces if anything happened to his DS.
If you continue to feel as you do, it will affect their sibling relationship. I remember as a child, I didn't like anyone my mum didn't like.
Even if she didn't show her dislike in front of them, we (siblings) knew how she felt.
Much as you may say or think you don't show it, feeling one way within yourself and having to outwardly display something else, causes internal conflict and you run the risk of it affecting your mental health the longer you feel as you do.
I have to do this with someone at work and I usually feel sick after speaking to her. I have to be nice and helpful, but because of who she is and how she behaves... it's very trying.
Sorry...this post is longer than I intended... but my message is your feelings need to change for everyone's sake... You, SS, your DH and your DS.