Picture Books About Becoming a Big Brother

Mercer Mayer's Little Critter book, "The New Baby" was our favorite, short, sweet and to the point.
Also, make sure you don't make TOO big of a deal out of the new baby, it can backfire. Keep the baby talk to a minimum and try to keep your son's life as normal as possible. Even though we parents see a new addition to the family as a huge deal (obviously!) young children just don't care that much, and they can easily be stressed out if you turn it into a big thing.

Here's one that looks very good with a lot of information, both from some biological standpoints (covering development in the womb, some reproduction, some breastfeeding) and in regard to what the baby can do, what changes a child might anticipate, etc.

http://www.amazon.com/Babies-Dont-Eat-Pizza-Brothers/dp/0...

If you have a sense of humor and want to let your son know that conflicting emotions are okay, here's one:
http://www.amazon.com/Darcy-Gran-Dont-Like-Babies/dp/0374...

"What Baby Needs" by Dr Sears and company also offers very simple, straightforward advice and guidance:
http://www.amazon.com/What-Needs-Sears-Children-Library/d...

Look up "Bear With Me" by Max Kornell. It's cute- the parents get a giant bear, without asking the "brother" and he has to adjust. A 6 year old would totally get that it is a metaphor for adding a sibling to the family.
The issues with the giant bear coming into his life are very relatable for a kid that age.
Congrats!

We just read one called Mail Harry to the Moon. It was cute and fun, but not super informative. Do you have a good children's librarian at your library? They're a great resource for things like this.
Good luck and enjoy!

My son adored Franklin the Turtle as a young child. There are stories for this scenario, and I'm pretty sure we might have had it/one. (also Berenstein Bears has a great one)...
But, we LOVED the DVD/movie of Franklin and the Green Knight. Franklin is a little put out by all the planning for the baby and attention to his mom over the pending birth, so he goes on a quest. It's really cute, and not too long (75 minutes). He takes his trusty sidekick along (snail) and has lots of adventure. To discover, that having a baby sister is really pretty nice in the end.
It isn't JUST about being a big brother, but that is a big part of the story. Franklin is worried about how he will fit in after the birth, what kind of big brother he will be, etc.
We enjoyed it a great deal.

http://www.amazon.com/Franklin-Knight-John-Van-Bruggen/dp...#

There are a couple books by Dr Sears that we got for our older 2 when I was pregnant with my third.
What Baby Needs
http://www.amazon.com/What-Needs-Sears-Children-Library/d...

Baby on the Way
http://www.amazon.com/Baby-Way-Sears-Childrens-Library/dp...

Little critter books are great. And Bernstein bear books