K.M.
I'm going to vehemently disagree with the previous poster. I don't think there's anything wrong with being a "pacifier" if you're both ok with it. It's not abnormal. Nutrition and comfort can be separate aspects of nursing. I think they're BOTH important. Toddlers obviously have a high need to suck. That's why you see 3 and sometimes 4 year olds running around with pacifiers not wanting to give them up. Nobody says their parents are "crossing a line."
When colostrum starts to come in, supposedly it's salty, so some toddlers don't care for it. So he may decide not to nurse anymore on his own. However, if your first still wants to nurse when the baby comes, there's nothing wrong with that. It's called tandem nursing. I've heard that tandem nursing decreases sibling jealousy. I don't have any personal experience with it though. I continued to nurse my daughter several months while pregnant with my son. But, I gently weaned her because I knew tandem nursing was not for me. She was 25 months when she was weaned.
www.kellymom.com has a forum for tandem nursers so you will find lots of support.
I also looked up this article on there for you:
http://www.kellymom.com/nursingtwo/faq/index.html
There's a link at the end for dealing with criticism. ;)
Good luck! And congrats on your pregnancy!