Do you know him well?
I would not even ask him. If you need to go inpatient...that's what you need to do. I don't think he can do anything about it one way or the other. If you call in sick you need not tell him anything except your calling in. He may require a back to work note, but that's about all that can happen. If you need it apply for FMLA through tour HR.
All the best!