This is my recommendation for your husband.
He needs to speak with his supervisor, not Casting, Traditions. His supervisor sets the schedule. He need to be clear about your treatments - how frequently, how far in advance they can be scheduled, are they typically on the same week day, how long do they take (include travel time to/from), does he need to be with you for a period of time after you get home, how many months will you be undergoing treatment, etc. And I'd also provide a letter from your doctor explaining all this He should offer suggestions for scheduling (I'll be willing to take the crappy shifts no one wants). If your husband has a highly skilled and in demand trade, he's in a better position to bargin.
I cannot believe no one in TDO has flipped out over a turnover rate that high...it's a clear sign that something is seriously wrong with a company when they have a 95% turnover rate. My god, personnel recruitment, training and retention is costing them a fortune!
If his supervisor is unwilling to work with your family's medical needs for this period of time, your husband may have to find work at another company, as much as he wanted to work at Disney.
Good luck to you both!