Hi S.,
Congratulations on your soon to be new family member!
There are so many great books out there for new siblings! Here are a few of my favorites that are appropriate for your son's age and developmental level: 1) Everywhere Babies by S. Meyers 2) On Mother's Lap by Ann Herbert Scott 3) I Used to Be The Baby by Robin Ballard 4) The New Baby by Mercer Mayer and 5) I'm A Big Brother by Joanna Cole.
Buying your son his own baby doll to take care of is a great idea.Don't forget to include simple props such as a baby bottle, doll diaper and blanket, rattle, and doll stroller for him to use in his play, as well.
Try to allow, encourage, and help your son to engage with, and help you care for his new brother or sister as much as possible from the beginning without "forcing" the issue- can he hand you a diaper, sing a song for the baby, bring a blanket, choose a toy to show or share with the baby? The more your son feels a part of the experience- the better.Be sure to thank him when he helps, and to point out how much his sibling loves him!
One other tried and true tip to ease this transition- make sure (no matter how tired you are- and you will be- trust me!) that you and/and or your partner spend some "special" alone time with your son every week, without the baby. It doesn't have to be a long time- half an hour is enough- but this is his special time with you, when you are completely focused on him, and doing something together that he enjoys -without the baby.
Wishing you lots of love and luck as you grow your family, L. S.