Hi! I have a situation very similar to yours. I have three step-kids: 12, 9, & 6. I have been in their life since the youngest was 18 months old. I tried very hard for about 2 years to be friends with their mom. I don't really like the woman, but I also had visions of one big extended family. She had other ideas - well, she still does. She hates my guts. And the kids are very much aware of that.
But I still have a really great relationship with the kids. In fact, the 12 year old is starting to get really irritated with their mom for talking so badly about me. I've always been very careful not to say anything negative about their mom when there's any chance at all they might overhear.
I eventually gave up on trying to be friendly with her and basically just ignore/avoid her as much as she does me. Apparently she has more than enough friends, and I have more than enough people wanting to give me grief - I don't need it from her.
It would be nice if she were able to see past her own issues and think about what would be best for the kids, but that's obviously not going to happen, and I don't lose any sleep over it. I'm still very pleasant when we have to interact and she's gotten better about being civil. If at some point she decides she'd like to have a friendlier relationship with me, great; if not, oh well.
I will again echo the pp who said to not bad-mouth her to your stepson (which I'm sure you already do). I know it can be hard to bite your tongue (BELIEVE ME, I know) but it's just not fair to put him in a situation where he feels like he has to choose sides.