What if your medical condition requires a longer absence than the 12 week FMLA allows?

My husband was hospitalized with a heart attack and after doing a heart cath, found that he would require open heart surgery. Because they gave him a blood thinner upon arriving at the hopital, they had to wait an additional 5 days to do surgery. This meant that he was 9 days into his FMLA leave even before surgery. Then, after surgery he developed double pnomonia as well as a staff infection in the leg they removed the veins from. It has been a very slow recovery and in 10 days his FMLA time will be expired. He really needs an additional 2-3 weeks for recovery but his employer has said that if he doesn't come back as scheduled, they will terminate his position and may or may not hire him back when he is able to return to work. Does he have a legal leg to stand on since it is only a couple of weeks that he needs or is he just out of luck? Thank you in advance for your answer.