I have exactly your situation. My son was 10 months old when we learned we were unexpectedly pregnant with #2. My son was 19 months old when my daughter was born (she was quite late), and my daughter is now 3 months old. Like you, I was completely terrified and caught off guard by this unexpected development. We had not planned on adding to our family nearly so soon.
First of all, you don't have to immediately wean off of the bottle at 1. 1 is a good age to BEGIN the weaning process - my pediatrician said she wanted my son completely off of the bottle by 15 months old, and recommended I begin weaning at 12 months old so that I had plenty of time to work it out. This worked beautifully for us because we had several months to gently back off of the bottle. Don't think of the first birthday as the deadline, but rather as the starting line, and it will help you relax.
Secondly, if your son is anything like mine, he won't have a CLUE what you're talking about with the new baby. We explained to him, when we bought a new crib and set up her room and everything that this was "Sissy's" bed, etc. (we didn't know what we would name her until after she was born), but it was obvious he didn't get it. But when Sissy came home from the hospital, he really just accepted her as a part of the scenery. In fact, right now at least, he completely adores her. We praise him a lot for helping (around 18 months old, they start to really enjoy helping, and so we make a big deal about having him bring us a diaper for her or something) and for being such a good big brother, and it's currently a matter of pride for him. Whether that will remain once she's old enough to move around or steal his toys is not yet known. ^_^
If you're like me, you'll be completely exhausted at the end of your pregnancy, so your son will have started to adjust to having less of your time and attention before the baby even arrives. This may work to your advantage, because your son will have to learn to entertain himself, but he won't make the connection between your inability to fully engage and the new baby who hasn't arrived yet. Once the new baby does come, make sure you spend at least 20 minutes with just your son every day. It is harder to leave a newborn for longer than that - as Baby ages, you'll be able to spend more time apart, but in the beginning, 20 minutes is probably all you'll have. Feed Baby, change her, then hand her off and read or play with your son. If she cries, comment to your son, "Ooo, the baby is upset," or something, but don't rush to her. Remember that she's fed and safe, and that she won't remember that you left her for 20 minutes, but your son will remember that you made time for him (and he'll also remember if every time she cries, you leave him).
I have little pieces of advice for you (and I'd be happy to chat with you in a private message if you want). First, if you have anything you really want done when your son is 18-19 months old, do it early, before the baby comes, or accept that it won't happen for a quite a while after s/he comes. I didn't wean my son off of his paci before our baby came, and now it seems like it will be impossible for a little while, because she takes a paci too and he's not inclined to give his up when he can see her with hers!
Secondly, invest in a baby carrier. I got a Beco Gemini for Christmas, and I LOVE it. I had a Baby Bjorn that I have been using and it worked well and served its purpose (but the Beco Gemini is sooo much more comfortable, and it's good for newborns, infants, and toddlers, so it will last me a while). If you are able to strap Baby on securely and then follow your son around with both hands free, everyone will be happier. You will be able to lavish more time and attention on your son, and Baby will be happy because she is up against you and your heartbeat, and feels like she is being held. Everything is sooo much easier this way.
Also, invest in a good double stroller. I have a Combi Sport Twin, and it has been a lifesaver. Shopping with 2 under 2 is an experience, but if I need to, I can strap them both in the stroller (my Graco pumpkin seat actually fits in the Combi so the baby can stay put) and head out pretty easily.
Is it hard? Yes. I am exhausted, and some days, getting anything done feels hard, especially with my daughter still so young. But actually, now that I have this baby, I'm not sure I would have done it differently. My son adores her, and you can tell she's really interested in him, too. I have hopes that they will be really close growing up. In the end, it all works out. Good luck.