Can you get fired....

Can you get fired from your job for having so many doctor's appointments? My husband is a war veteran and for the past few months his doctor in the VA hospital has been scheduling around 3-5 appointments each month for follow ups and testings, and aside from that, he is also attending therapy for PTSD once a week. Yesterday his boss and the director had a meeting with my husband, and they gave him a warning because they feel like it's too much. I'm scared that he might lose his job for missing work hours due to these appointments, even though he works overtime to make up for it.