Yes but my son is 27 and has ptsd and paranoids skitzophrena and a mind of a 6 year old also needs 24 hour care and she refused to sign paperwork for his dad

Yeah, that sucks, sad story.

Maybe if you ask the physician can it be done for a fee, the answer might change. HINT HINT HINT
You may need to try a different doctor. Is the doctor refusing because she doesn't want to be bothered? Or is she refusing because she doesn't think the situation qualifies?
There isn't any law that is going to force a doctor to complete FMLA paperwork if they don't feel it is warranted or for some other reason does not feel it is appropriate.
There is no law that says the dr. must fill out FMLA paperwork. However, did the dr. give a reason why she will not fill out the paperwork?
Did you ask the doctor why? If your sons mind is at the equivalent to a 6 year old, the doctor may not think fmla is a good thing for him. Speaking from experience of someone I knew in a car accident that injured their brain and basically was a child in development again. Working maybe the best chance for your son to regain mental stability again. Like my friend, the rehabilitation was using there brain with challenges much as possible. There's a lot of reasons why the doc may do this. As for FMLA, it's really up to the doctor.
Sounds more like Dad (the employee) wants FMLA to care for an adult disabled child. FMLA is only appropriate as it relates to the serious health condition and treatment. It is not "day care"
I thought this was where you can get small legal advice from real layers,?

Legal advice can only come from your lawyer, not just any lawyer.
I am a lawyer, but rarely dispense legal advice on this site.
If I choose to do so, it is never done publicly.
That said, if it is legal advice you seek, HIRE yourself a licensed lawyer in your county.
There is a disclaimer about legal advice and what this site offers.
We are all volunteers.
We try to help people by making suggestions, discussing issues, pointing out the relevant law that could be applied to their dilemma, but we never advise anyone as their personal lawyer.

If you find an internet site that claims to dispense legal advice, be very wary, and be very cautious.
Do you have legal guardianship over the disabled child? that is have you gone to court to prove he is unfit and needs a guardian? Has the 27 year old told the doctor that he doesn't want his parent(s) involved in his medical care? It is possible that your son needs to request that it be filled out.

there are way too many questions to answer here. I suggest seeking out a local attorney who is familiar with guardianship of disabled children (and how seek and get that).