Hon, I'm sorry you are in that position.
I can suggest you perhaps try to enlist the help of "experts" against your stepdad, if you think that will help.
Unfortunately, it sounds as though your stepdad is the kind of guy who will do as he pleases, regardless of your wishes.
Are you in a position to make other living arrangements?
If not, you may wish to try any (or all) of the following:
~ Enlist the help of your child's pediatrician.
~ Purchase a child development book and use it as your source of proof to back you up. Be aware that experts have varying opinions - read through the books and select one that best meets your ideals before you buy. (I like John Rosemond, myself.)
~ Take some Early Childhood Development classes at your local community college.
~ Discuss your questions with other experienced moms. Here is a good website (by moms, for moms) where you may get the support you seek: www.breastfeeding.com.
Most of all, have confidence in yourself. Parenting, like many other things, is a learn-as-you-go experience.
If you can set limits and stick to them, love your child and teach your child what you want him to know; you're ahead of the game already.
Sweetie, I commend you for seeking the answers and for taking a stand and trying to do what you feel is best.
I think you and your child are going to be just fine.