Why would you want ADA to accomodate a potentially dangerous situation? You said yourself you would like to keep your husband's injury stabilized. Knowing how much he has suffered, why would either of you inadvertently want to hurt a fellow EMT or patient, putting them through what he has experienced? Factor that in with the probability of a patient dying because he was waiting for additional help.
You also may face the prospect of having your ambulance shut down because you didn't have a crew to go to the call. It has happened to many a volunteer agency. Frankly, I am suprised your ambulance is willing to take your husband on, regarding their liability insurance. Suppose the ambulance is involved in an accident and your husband is paralyzed? Who is going to pay that bill and settlement? Who would you expect to take the ultimate responsiblity? There are so many factors here and it is reminiscent of a bad train wreck waiting to happen.
As his wife, I would suggest you encourage him in other areas where he can function fully and without worry. There are some excellent programs in which people who have injuries like this can gain employment by entering a work program in which he would be trained for a job that is better suited for him. In some instances, they will pay for additional education, if needed. ADA might be able to direct you to those programs.
I think you know in your heart this is not an ideal situation and what needs to be done, or NOT done, in this case, in the interest of your husband, his partners and his patients.