Congratulations on your pregnancy, OP, but I agree with others. Your DH should have at least had the courtesy to warn his ex so that their 12 year old child didn't have to be the one to break the news. You may have had the impression that your stepson was excited about the news - and he may very well be - but how do you know he didn't go home to his mum and express confusion or fears about being pushed out of his Dad's affections because of the new baby...?
My son was 5 when my ex married his very lovely, brilliant stepmother, of a wife. My son and my ex's wife have a fantastic relationship, full of support and kind friendship. They told him they were getting married and he was full of glee for them... until he came home when he cried and begged me to still be his Mummy after his Daddy had married his stepmum. You don't know for certain - and probably never will, because once I'd reassured my son that yes; I'd still be his Mum, I restrained from sending a bitchy text to my ex's then-girlfriend (whom I'd only found out about a few months prior when she gave birth to their baby... literally on the day she was born, was the first my children or I knew about her or the then-girlfriend) about it.
What matters here is that this isn't about you or your impending baby. It's about your DH and his son, and their relationship. It's also about the ex, I'm afraid, who will be the one who is possibly doing her utmost to (a) conceal her own shock at the fact that her ex didn't have the manners to give her a heads'up so that she could support their son appropriately, and (b) might even be doing her utmost to tell that son that of course his Dad will still love him after the baby's been born...
I'd imagine that after 11 years of your being on the scene, she'd grown used to the idea of her child being your DH's only. That's changed now. It will have been a shock to her system. And she'll be angry that her son was placed into the awful position of having to be the one to tell her.
Hopefully the baby won't be born on your stepson's birthday... but they come when they want to, I'm afraid. My son was supposed to have been born a full 33 hours before he actually was (I was induced) - but if he had, he would have shared his birthday with an uncle of mine who died at 6 days old, so I'm actually very grateful that he stayed put until the next day! Like others have said, and I'm sure you agree, your baby and your stepson each deserve their own day where the Dad they share can spoil them appropriately.
Be kind to your DH's ex. I ended up babysitting my son's younger half-siblings (for free, I might add) so that he and his now-wife could go out together, and I have them after school sometimes when their parents aren't around. The ex? May well turn out to be a blessing in disguise... particularly as you already seem to have a friendly co-parenting relationship with her, as her child's stepmother.
But don't put a child into the middle of what is, at the end of the day, an adult situation.