K.,
Just my two cents...
I am a co-sleeper, and no it's not for everybody...so, I respect your need for getting some decent sleep while pregnant. Even as a co-sleeper there is the need to be able to get up, and do some laundry or read a book. So, this is what I did to 'wean' my son off my presence at night...now, it took us about four weeks total, but it worked for us.
1. Keep up your routine, it sounds like a great one.
2. Keep talking to your son, kids deserve respect, compassion and to understand the need for change.
3. Introduce a 'lovey'...my son and I made a special trip to the store/mall to pick out something that he would be 'in charge' of taking care of and loving at night.
4. At bedtime, after routine, I told him Mommy would hug him, and he would hug his 'lovey' that was a cuddly dinosaur he and my sister picked out when we were shopping. Then, I told him I loved him, and that while he slept Mommy had to go take care of things in the house. It was his job to 'take care of his dino' and make sure he was safe at night.
It took us two weeks of this, and fussing and him saying 'Mommy stay'...but, I would wait until he was sound asleep and then, leave. I made sure to keep a baby monitor with me if I wasn't near our rooms, and come to him if needed me. I would pat his back if he needed it, and then he'd go to sleep again.
5. In the morning, make sure he knows how proud of him you are. Kids love to be rewarded for ALL the good things they do. Special treats or boards where he can earn a treat are great ways to encourage transition in toddlers.
It's a great thing you've done...meeting the needs of your child, and making sure he knows he can count on Mommy no matter what. One thing I just thought about was making him a part of the baby's world now. When my Mom was preggers with my sister, she had me rub her belly and 'help' her with getting things ready for the baby. It made the transition to being a 'big sister' easy and fun for the whole family. Explain to him to him what's going on with you, and that you need rest so the baby can grow...just like he needs sleep to grow.
Keep in mind he will get sick again, and he will need you for love and comfort. If you transition him now, future colds and such will be easier to get through and then go back to regular routine.
I know the whining and crying at night is tough, but it's just a sign of an amazing bond that he fears will break if you go. Make sure he knows it won't, and that Mommy will always come back IF you NEED me, but that Mommy needs to be able to 'take care of the baby', the house, the laundry whatever.
Best wishes,
Deanna