Hi! Congratulations! First, La Leche League has a book that is helpful: Adventures in Tandem Nursing".
My kids were a little closer than yours and it worked (and is still working) just find for us. I was 9 months when I found out that the last period I had wasn't a period and I was 10 or 11 weeks along. Finding a supportive doctor is key. I went to a check up with my doctor and said something about nursing my son and she said "and that's fine, you can keep nursing as long as you want." I said "That's good, I wasn't going to ask." She told me some doctors don't think it's ok...sure, doctors who haven't read anything on lactation in the past 20 years...In any case, fighting over something like this from the beginning of your pregnancy is just going to stress you out, so make sure it's not an issue with your doctor.
First, your milk supply may dwindle or even dry up around 6 months and this might make your son mad. Your breasts may become very tender and you won't want to nurse at all for a while. Your milk will change at some point. Mine never went back to the "gold" colostrum but it definitely wasn't the same as the milk my son was used to - he couldn't have cared less...your's might.
If your son wants to stop, let him. He may be interested again once your milk comes back in and he sees the infant nursing. You can nurse one on each breast or nurse the baby first and then the toddler.
The two best things about tandem nursing for me were 1) I was never engorged, I could just offer to nurse my older son and he'd take care of it! 2) when I was in labor with #2, I kinda stalled out and they started talking about breaking my water and then pitosin which was totally against my natural birthing plan (there's more to that whole story if you want to hear it) so I told the doctor (on-call, not my own who was out of town) that I would nurse my older son and see if the nipple stimulation helped. I figured if nothing else, at least it would buy my some time before I had to start fighting with them over statistics of augmentation. Luckily, my contractions went from every 6-7 minutes to 2-3 minutes!
Here is my advise after going through it...
When people ask why you haven't weaned the older one (or when you will, or what you will do when the baby is born), a short explination is all that is necessary. "The AAP, WHO, and UNICEF all agree that the second year of nursing is as beneficial as the first. I didn't want to take that away from my son just because I am pregnant. He'll let me know when he's ready."
Don't worry if you aren't gaining weight. Your body will take care of the baby if you take care of yourself. Eat a healthy diet and keep water close by at all times. You'll be fine.
Nap when your son naps. I have never been a napper, but I was unbelievably exhausted at the beginning of my second pregnancy as I tried to keep up with my son (and yours is probably already walking!).
Don't take it personally if your son does self-wean. Your milk will change and he may just not like it. It has nothing to do with you. And, like I said, he me want to go back to it later.
You will have contractions as you nurse but you won't go in to labor unless your body is ready. Don't worry about the contractions, they are harmless like the ones you might get after you pee or have sex.
This was probably longer than you expected. PM me if you have any questions. At this point I am nursing a 3.5-month-old around the clock and a 20-month-old in the morning and at bed time.
Good Luck!