It could be due to the medications, the depression, the chronic pain, diminished sexual ability post spinal injury and surgery, or a combination of all of these factors.
It might also be due to his concern about pain with sexual performance or his fear of inability to perform in the same way as prior to surgery. The initial spinal problem may also have residual effects on his sexual function.
In addition, his lack of interest and function could be due to a primary urologic problem.
I would recommend trying to discuss this with your husband. If he is resistant to discussion, you may consider encouraging him to discuss this with his doctors.