While we only have one almost 3 yr old son right now and no baby on the way, he loves his doll. I took him to the toy store awhile back and told him he could pick out one toy. What he wanted was a doll and baby bottle. He would feed her, named her, "changed", and put her nightnight. I don't see a problem with a little boy having a doll at all. It encourages gentleness, compassion, and a sense of responsibility (of course he did that himself, we never told him he had to care for her).
It would be a great way to practice for your son. Then when the new baby arrives, you will have your baby and he will have his.
Although, I recently heard a story where the older child, 2, started treating the new baby like a doll, trying to pick her up and even sat on her when they weren't being watched (of course, they should always be watched!)
Good luck