You'll get a lot of 'you should have known', OP, but the fact is you didn't, and you're here now.
For me, something shifted when I stopped trying to play 'good stepmother mode' and started being more myself. I realised that to some extent I was trying to act a certain way when my (former) DSS was around, and always have him be the holy centre of everything (as his dad was understandably happy to do) and I just stopped.
You're allowed to show up as yourself, and you'll probably add more to his life by being more yourself, because he'll get to learn and be influenced by a different kind of person. And that's actually much easier when you have a lovely, polite, switched-on kid to work with, as you do.
So if a conversation at the dinner table is boring for you, change the conversation. What do you want to talk about? And how do you get your DSS involved in that chat? I remember I bought one of those conversation card packs – they were questions that were designed to spark a bit of debate, and for a while we'd pick one every Friday night to talk about. They took us down some pretty interesting and often very funny chats.
Or if DSS wants to talk about 12-year old boy stuff, ask him questions that would actually be interesting to hear about.
When my DSS was about that age, Fortnite was his only interest, and I would also want to space out with chat about skins and llamas.
But I started asking him about his thoughts on talking to strangers on gaming platforms and what he would do if someone joined a game that he didn't like, and we ended up in some good conversations about ethics and social behaviour.
He's now 19 and still remembers some of our dinner table conversations. 12 year old boys are 12 year old boys – but they're also capable of a lot more than we sometimes give them credit for.