My DSS is 8 and our first joint DC is due in September.
We told our families at 12weeks once we had a scan photo and then told DSS at about the same time (he lives with us). DP sent a text to exW after DSS was told just to keep her in the loop (she even said congratulations!!!
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DSS was actually quite upset when we first told him, as in buckets of tears, but once we worked out that he was mainly bothered about the baby interfering with his lego, the words 'you'll never have to share your toys' was an instant calmer. He is now really excited about it but has solemnly told me he would prefer it to be a boy.
Being the step-parent within the dynamic, it was really important to me that DSS felt included but also to have it understood that he is a child and as such doesn't really get a say in what happens - ie sibling will arrive whether you like it or not and things like names - he can suggest ones but ultimately myself and DP are the parents and will make the decisions. When we told DSS very coincidentally he had just asked about 'where babies come from' so literally the week before we announced the pregnancy we had bought him a book and were answering lots of questions.
As your DSS is a teenager I can't really advise how to approach it, I would probably tell him first after 12 weeks (its much easier to show a scan picture to make it 'real') IFSWIM. Rather than say 'we're having a baby' which might make him feel excluded, maybe say it like 'you're going to be a big brother'.
However you approach it, just be positive and upbeat, and congratulations!!
what exciting news for you