It's best to stick with baby soaps and washes for a while. Regular soap contains heavy surfactants, which create that soapy lather; deodorants, which eliminate body odor; and fragrances. Babies and young children don't need these things, and they can actually irritate their skin. Using a regular adult soap on your baby's silky skin will likely leave it dry, red, or splotchy. In fact, your child's better off using baby or children's soap or mild moisturizing soap until puberty, especially if he has sensitive skin.
Remember, most pediatricians say you don't need to wash your baby daily -- only about three times a week until he turns 1. And during bathtime, you don't need to lather your baby from head to toe; just use soap on the areas that actually get dirty and sweaty, like his bottom and privates, armpits, and knees (you can rinse the rest of him with only warm water).
Of course, if using one type of bar soap is more practical and economical for the whole family, then switch everybody to a gentle, unscented soap, like Dove, that's safe for the youngest members of your brood.