My husband has a herniated disc. Should he sleep on his back?

Not neccessarily . I have unsuccessfully tried to make a science out of sleeping positions and mattress types for various conditions. Sleeping on the back is usually best for herniated discs- particularly if the disc is not all the way out. Sleeping on the back places the back in extension and reduces many discs. If the disc is fully extruded, he may be more comfortable on his side which will provide more room.
Yes. The most common positions that people with herniated discs find comfortable are lying on their back with a pillow under their knees, or lying in a fetal position with a pillow between their knees.
Yes. Depending upon the location of the herniation, sleeping on one's back may bring relief or may make the pain worse. This decision should be made based on symptoms.
Yes. Someone with a herniated disc may sleep on their back, side or stomach. They should sleep in the position that is most comfortable for them.