Your body will "reset" for the newborn, I believe, so the colostrum will come in (and you probably don't want to have him get much of that, since it has a laxative effect to help the newborn pass the meconium).
Try googling "tandem nursing" or contacting your local (or state) La Leche League. Most large urban areas have a LLL person who is experienced with tandem nursing and can help you.
I will tell you that a couple of moms I know who had plans to continue nursing the toddler after the baby was born actually had their little ones self wean--the milk changes in taste due to hormonal changes of pregnancy, and the toddlers said the milk "tasted bad" or "tasted funny" to them. The one lady I know was nursing her twin boys, age 2.5 when this happened. She had been telling them for a while that "when you are big boys, you won't need momma's milk any more" and so when the milk tasted funny to them, they decided they were "big boys" and there was no fuss at all. The other mom didn't say whether her little one was upset or not over the change in milk/weaning.