Talk about siblings, get books out of the library about the transitions involved in new babies, get books or videos about pregnancy and babies.
Don't make the mistake of telling him that he's going to get 'someone he can play with' because it will be years before the baby is capable of playing.
Don't make the mistake of doing 'big boy and mommy only' activities, because that will imply to him that the only way you can love him, really, is for the invader to be somewhere else. Engage in activities with the baby in your lap, or wearing the baby, so he gets used to the idea that it's not all-or-nothing attention for him.
Do tell stories about him as a baby, and what was involved in his care, so when he sees you doing the same things for his sib, he'll know this is normal not mommy loving the baby better. Go through his baby book, look at pictures of him as a baby...
at 9:09 PM on Oct. 31, 2010