Most of the time, spinal bifida would not affect sperm production. However, it could cause problems mechanically or neurologically, which would result in difficulty during sexual intercourse.
As long as your husband has no disability in erectile function and does not experience significant amount of pain in his lower back during sexual intercourse, then there should be not much concern in relation to fertility.
If he experiences lower back pain during the act, maybe he should try for a different position.
Lastly, it is a good idea to have radiographic study and MRI study for the lower back because these imaging studies would help you to find out if any potential mechanical or neurological concerns exist, i.e. intervertebral disc herniation, spinal instability, etc. Fertility test (sperm count) is useful as well. Hope this helps.