Americans with disabilities act...Can he be fired?

My husband has recently been called into his employer's office because he refused to work during a day shift without having several weeks notice. My husband took a third shift job because he has ulcerative colitis and I am legally blind. The only way we can get to our regularly scheduled doctor appointments is for him to work at night. He doesn't mind and it works out best for us. Now, his supervisor wants to switch him around every now and then at a moments notice. They switched him to 1st shift for 2 weeks. They are also saying that he cannot use my blindness as an excuse for not working 12-14 hour shifts without prior notice. We are wondering if it is legal under the Americans With Disabilities Act for his employers to fire him or take disciplinary action against him because he cannot work days without more than 24 hours notice? Not many people volunteer to work a third shift job and his boss knew he took the job because of our disabilities and need for regular doctor visits