S.,
I am a mother of three who is expecting our fourth in December. My first two children were 21 months apart. At the time I was expecting our second child, I was also worried about how to deal with two young ones at the same time. There are a couple of pieces of wisdom I would like to pass on from my experience.
First, if you haven't told your child a new baby is coming, now is the time. There are a lot of cute books about new babies in the family for children. There is one in the Berenstein Bears collection that I know of. Second, if you are planning on putting your first child in a big boy bed you should start now before the baby is born. This allows the child to believe he is being moved to a big bed because he is old enough, not because the new baby kicked him out of his crib, which can cause resentment. If you are planning on just getting another crib then there is no need to put him in a big boy bed yet. We went ahead and put our oldest in a big bed 3 months before the baby was born and he loved it.
If your son somewhat understands that there is a new baby coming then you have prepared him as much as you can. Don't worry too much about it, most likely he will be like my son was and be a little curious about the new baby, but not really interested. He ignored her and played happily with his toys. Two things I would suggest about this transition time. One is that if at any point, both your toddler and your baby need you, let the baby cry a little bit until you can tend to the toddlers needs. The baby won't remember that you didn't tend to her needs right away, but the toddler will. And second, get a little gift for your son from the baby when it is born. This will help some with the transition. The hardest part for a toddler when a new baby arrives is not receiving enough attention. If he is getting his needs fullfilled in the same way he always does, the new baby will not be a problem.
Finally, I will tell you that it takes some time getting used to having two children around, but eventually you adjust and learn to organize your time. Utilize your husband, your friends and your family, let them help as much as possible, at least for the first 4-6 weeks until you recover from your c-section. For me, once I got used to the baby's schedule it wasn't so bad. The worst part of having two that close together is when they are both mobile. They become busier and will tend to run different directions at the same time. So be prepared.
I hope this helped you a little. God bless you and your new baby!
S. B