Hi K.F.,
I haven't read all the responses, sorry if I duplicate some.
I remember when I was at about your stage, I was feeling so sorry for the new baby even before its birth, because I was afraid I could never possibly love him/her as much as I loved Benjamin (then almost 3.5 yo).
I did take Benjamin to the zoo 2 weeks before the baby ( Philip) was born (people thought I was nuts, zoo was 2 hrs away), and it was one of those "million-dollar days" for me anyway, we had such a good time. (Don't know if Benjamin remembers it, he is 19 now). So I would do as one resonder said, go someplace special and have the photos out. We didn't have the photos out but we could talk about it. I would just do lots of things with him so you can talk about it. And enjoy his company, try to "soak up" what he is like at this age.
I also liked the idea of getting him used to playing by himslef for a few minutes at a time. But if he is not up for it don't stress over it.
A lot of people seem to think that the older one is "left out" when mom is feeding baby, esp. if mom is nursing (which I hope you will do , but that is a topic for a whole other post!) This need not be the case! You can ask him to bring a burp rag for baby, a cup of water for you (maybe a sippy if you are afraid of spillage--lol!), a blanket for baby; stroke baby's feet if baby is falling asleep and needs to nurse more, etc. Or you can quietly read him a book while you nurse (he can hold it), or (once you can nurse one-armed) just put your free arm around him and cuddle and talk. All this stuff works for bottle-feeding too.
Going from one to two--your laundry will double! or it will feel like it, even though you are only adding one person--I have talked to several friends who agreed that it sure seemed like the wash doubled every time they added a baby!
Check with your hospital to see if they have a sibling preparation class.
Let your son pick out a toy or book for the new baby. We also got a toy for our older one and told him it was from his brother--eventually he wondered just HOW the baby got it for him, lol!
Ask you children's librarian for books to read about a new baby arriving. I can't remember any titles offhand.
There is a very nice book called "My New Baby and Me: a First-Year Record for Big Brothers and Sisters" by Dian G. Smith. It is a book where the big bro or sis answers questions and fills out the pages (with adult help) about what s/he can do and what the baby can do. You could get it for your son or ask someone else to give it to him--with the baby getting lots of gifts, it is nice for the older one to get some too.
Take some photos of your son, maybe of him sitting on your pregnant lap; it's easy to get in the "take lots of baby photos " mode.
There is definitely an adjustment for your toddler because he has always been the center of attention, and hasn't had to share you with anyone else. For an interesting take on sibling rivalry, (and I'm not saying it will happen right away, or at all), read the book "Siblings Without Rivalry" (I forget the author). The first part has a very good analogy that really puts you into the shoes of the older sibling.
I remember another mom talking about the family adjustment as well--because you are going from essentially a triangle, a very stable shape (remember geometry/ 3 non-colinear points determine a plane) to a square, a not-so-stable shape (think of how a 3-legged stool doesn't rock like a 4-legged one can). Now eventually you get the 4-legged stool stabilized, but it is not always a quick thing. (hope that made sense).
And fwiw, a friend told me "you only bring a baby home to an 'only child' once". Which is small comfort to those of us who only have 2..(she had 4) ..but oh well!
I don't know if this has been any help or not. Just know that you will manage. Enjoy being a "mom of one" for your remaining time, and look forward to the new adventure of being a mom of two!
And BTW, I do love my "new baby" (now 15.5) as much as my first! ;-)
K. Z.