First let me say that I hope that your. Dad makes a full recovery. Now with that said a lot will depend on the company that your Dad works for, they will most likely send him to a Doctor of their choice to be evaluated. This will be once his Doctor clears him to return to work.
Unfortunately, this industry is 100% or nothing when it comes to working on board any vessel. I myself had to retire due to having a back injury followed by back surgery that did not go very well.
I realize that your Dad is very worried about his job, but he really need to concentrate on his health and recovery. I hate to say it but whether he can return to work or not is kind out of his hands and up to the Company and the U.S.C.G. (if he will be on any medication when he tries to return to work).
Tell your Dad that we wish him a Full Recovery.