Nonunion means it didn’t fuse at one level. The X-ray probably showed a gap between the bones where the graft didn’t form solid, fused bone.
If dynamic X-rays are taken of the spine moving then motion would be seen at that level. With a successful fusion there should be no sign of movement between the two vertebral bones.
The spine may still be stable if the fusion is solid at the other two levels. Bone healing can take up to a year to complete. Your husband should make sure he goes to his follow-up visits. The final outcome may not be known until 12 months.