I'm not sure how Fed leave works, but also clarify if this is personal leave or sick leave. Some organizations don't allow sick leave unless you, personally, are sick, and the time off / payment scale can be different. I know where my DH works, this would be considered personal leave because it is planned, and for another person.

When I was at the Children's ER with my DS this spring, they gave me a note for HR as part of his discharge papers.

I had to get a note from my doctor when I had a procedure that said I needed X number of days off.

I agree with PPs, just ask the doctor to write a note that your son will need care for X number of days after the procedure.

Doctor here and I write these all the time!

This is a weird question but anyway....

Obviously you can't write the letter so you go to the MD and say my HR dept is requesting that I provide documentation providing why I need to be out of the office for x # of days.

Get the note from the dr doing the procedure. Just tell them you need a note for work - I'm sure that while this may be new to you, it's routine for them and they know what to write. It'll probably say your immediate family member needs full time care for x number of days/weeks.

I would get something from your son's doctor stating that for medical reasons he will need an adult present at home for X number of days. Or something along those lines.

I need to submit documentation that I will be taking care of my young adult son after a procedure. HR wants some sort of documentation for several days sick leave. It seems weird to get a note to cover that but I guess people do it all the time. I never have in my career but this is a new Fed job. What do you think they want and from whom?