How do you get a loved one to go to hospital?

My dad is 87, he is visiting my sister in Florida now. He left a month ago and 2 days after he got there he started with what we thought was the stomach flu. He drinks very little water in the first place but now he was throwing up just about every sip and then some. He ended up in the hospital for 3 days on IV and then sent him home after fluids. Now back at my sister's he still can't hold anything down and had diarrhea. He refuses to go to hospital or see any primary care doc til he flys home. His apt with "his doc" isn't until 3/18 and i fear he won't make it. I wonder if this has been going on for a while and he just never told anyone. He insists he is fine but he can't live on amount of nutrition he is getting. Anyone been through this?