Glad your husband is doing well. Great news! I agree with others that the onc has to cover his derriere and could not give your husband permission. I think it is very important that your husband have something constructive to do and, for me, that is the primary consideration. As someone suggested, if you take a trial run with him and see that his reaction times, alertness, etc are okay, I say, go for it.
When Lucie got off chemo last year and started feeling well, I thought it was important for her to be able to drive short distances as it gave her a feeling of normalcy and gave her things to do for herself. She has been on morphine ever since chemo, but it is the lowest dose that can cover the spinal pain. She started out with just trips around the block until I felt comfortable with her driving. At first, she was a little awkward, not having driven for months, but she came around to her usual good driving in a short time.
Now that she is back on chemo, she is not driving. But as soon as the treatment is over and she feels up to it, I will let her drive again. It gives her something she can do for herself, once I feel she has good reflexes and alertness. She does not drive long distances, as it is too tiring on her.
Find a way to get your husband involved in meaningful activity. That is so important to good mental health. Best to you. Don