My grandmother only bathes my grandfather every 4-6 months and is neglecting his care. How can I report this to social services?

My grandfather is 90 years old and lives with my grandmother who is 87. She is his sole caretaker, as I live far away and the rest of my family is not involved. I'm worried that she is neglecting his care. She only bathes him to see the doctor, about once every 4-6 months, and that is the only time he leaves the house. The last time I saw him he was caked in filth, with long fingernails and brown dried crust all over his skin. He just sits in his armchair all day in the dark, and sleeps there at night. He used to be so active and social, I'm worried he's lonely, bored, and depressed.

He is definitely not cognitively "all there," but no formal diagnosis has been made. He knows his name, where he is, and what year it is.

I think I could make a report of self-neglect/abuse to social services in the county they live in, but I'm not sure where that would get me. If a social worker came out and asked him if he needed a bath, I'm sure he'd say that he's fine and doesn't need one.

Any suggestions what I should do? My grandmother won't allow anyone into the house (like a home health aide.) She insists on doing all the driving, cleaning, cooking, etc. Basically I want to get social services behind me to force them to hire an in-home caregiver, but if my grandfather refuses, do I just have to let him stay like this? They have the money, they're just sitting on it.

Thanks.