I really feel for you SuziDee. My ds was born 10 weeks ago and my dd (then nearly 20 months)found it really hard in the first few days\week. She totally lost her sparkle and I felt very very sad that we'd seemingly removed her stability. She'd constantly point to the baby's carrycot in the corner of the room and say 'take it away'! However, 10 weeks on and she is completely back to normal and has been for a while. Just don't expect it to happen overnight, don't push them to accept this new interloper and continue to give them all the love you can while trying to keep their routine as normal as possible.
We think dd started to come around as soon as she realised that actually nothing had really changed in her little life. We continue to put her first if at all possible (leaving ds in his cot/bouncer and seeing to her first so she doesn't hear 'in a minute' from us constantly) and although we give her unlimited cuddles and reassurance we have remained consistent on rules and boundaries so that she feels secure.
A couple of things - don't rush your ds to engage with the new baby if he doesn't seem interested. Our dd totally ignored ds until a couple of weeks ago. Fine by us - it's not as if new babies are particularly interesting (at least not neccessarily to a toddler!). Slowly but surely, however, she's started to talk to him, kiss him goodnight (without us suggesting it), include him in the stories she tells, etc. It's lovely to see.
The other thing is don't be concerned if your ds rejects you. I was prepared for our dd to reject dp because they usually reject the one they can have in favour of the one they can't, don't they? Not in our case! Dd would have very little to do with me (going so far as refusing to cuddle me and one afternoon after being returned by the in-laws after a day with them, completely blanking me and calling for dp instead). It lasted about 5 days and left me feeling sick with despair. But, again, without pushing her to acknowledge me or insisting she cuddled me she gradually came round. I made sure we spent some quality time one-to-one and now our relationship is, if possible, better than ever.
Just give him time, love and consistency and he (and you!) will be absolutely fine.
Congrats, by the way!
Oh, and I heard a child psychologist compare the arrival of a sibling to your partner bringing home another woman and you being expected to not just get on with it but love them as well! You'd need a bit of time to get used to that, wouldn't you...?