If my 2nd son had hemolytic disease of the newborn, what are the chances the rest of my babies will as well?

Depends: Hemolytic disease of the newborn, which is due to rh or ABO incompatibility, happens when the newborn's blood type is incompatible with the mother's, and the mother has passed antibody to the baby in utero. In ABO disease, the mother must be type o and the baby either a or b. In rh disease, the mother must be rh- and the baby rh+. So, subsequent risk depends on the fetus' blood type.

For normal child: Get vit k upon delivery as per any newborn routine care. It will decrease chances of hemolytic disease of newborn significantly. If you are concerned about hereditary disease i would recommend to pay a visit to a hematologist and genetics prior to become pregnant with the next child.