I'm six weeks pregnant with my second child and my first child with DP, who is step-dad to my DS (10). I have been feeling increasingly sick and am now working from home/off work, and have been conspicuously absent from several things relating to DS (football matches, etc).
DP and I felt DS was owed an explanation, so we told DS a few days ago, gently but with a "this is positive news" vibe. He was amazing at first - took it so much better than I could ever have imagined. It was as though a whole load of primal instincts - love, protection, care, loyalty - bubbled up and he was so positive, saying he wanted to teach it to play football and to read, and asking if it would look like him, and hoping the pregnancy would go OK. He said he was positive about it "because it's family". He has a really good relationship with DP too, which maybe helped him take it so well.
However, a day later - yesterday - after a long day at school and clubs (tiredness may not have helped), he came home a different child. He is so angry, feels so rejected. He doesn't understand why we've chosen to have another, other than to replace him; he says we have our hands full as it is (DP has his DCs too, who are with us about a third of the time). DS feels he has no value anymore, doesn't see the point in anything - school, etc. He says I'll have no time or attention for him, and said this "fucking baby" has ruined the rest of his childhood. He also referred to it as "that piece of shit growing in your tummy" (he never talks like this normally, so please don't think he's so very awful), and said he wished I'd have a miscarriage. 
At this point, I walked out of the room and left him to his own devices until this morning (when he was still miserable and angry). I have tried so hard to be understanding - saying I know it's an adjustment, that some things will change but we think positively, that he can carry on doing all the things he loves. I've reassured him I love him to bits and always will. But I have also said he crossed a line saying what he said last night, and that he's not the only person in the household with feelings - that I don't expect him to verbalise any thoughts like that again and I expect an apology. I know he's going to feel all over the place, but surely there has to be a line somewhere?
I just feel so low about how DS is taking the news. DS's initial reaction seemed too good to be true, and perhaps it was. I'm so worried I've damaged our close relationship. Do you think DS will come out of this unscathed, eventually? How do I support him as he comes to terms with the news, listen to his feelings and help him feel reassured, without crumbling under his guilt trips and cutting remarks? How understanding-versus-tough is it reasonable to be with him when he's just had this news dumped on him? I'm listening and listening and understanding, and then after all the worst-case-scenario negativity, I want to just say look, stop trying to make me feel bad, and you'll get out of this what you put in. But I don't imagine that would help much!
Thanks so much for any support with this - and any good news stories where a similar situation has come good?!