Can I enlist the help of Social Services if my father-in-law refuses outside help?

I would call social services first some of the people in APS think they are more important then they are I was reported by a home care medicare nurse because she just did not like me -to APS and they sent out a big bully of a women who indicated she was a social worker -who it turned out was a case worker with no training and she harrased me something terrible saying I was abusing my husband and he likeing attention played into her scheme she finanly left and I called my daughter who worked for social service and she told me that women has no power and just wanted to cause trouble -later on I told someone this story and he was able to have it investigated-if you call APS and get someone like this they can try to make trouble. Social service should be able to direct you on how to get help. Guardianship can be done with the help of an E lder Lawyer and if he is has not bathed in 5 years he is not competent to care for himself that is one of the first signs of demetia. I would do it before someone reports you to APS like a neighbor or mailman or such.

You mention pallative care and hospice. Do you feel he is in the final stage of his life, that he is dying?

There should be an "adult protective services" organization in your town. Contact them for direction.
If he is of sound mind and refusing help I do not know if you can do anything for him. The only other route, and it is a complicated one, is to declare him incompetent and a danger to himself and seek guardianship. This is often a traumatic process. So I would try protective services first.
good luck