How bad is your DH's injury? Will he be able to represent himself in cout? You can fill out all the paperwork for him. You can serve the other party the paperwork. You can file the proof of service with the court. But it is DH who will have to stand in front of the judge, talk to the judge, explain the circumstances, and request the modification.
If he physically is not capable of appearing in court, then you have a problem. If you prepare the whole motion and then go talk to a couple of attorneys, sometimes they won't charge very much to stand in court and argue the case for you.
