I badly need your help!! Father refusing to drink water

My dad, 78, is suffering from several diseases (diabetes; gangrene due to diabetes; the loss of vision due to diabetes; hypertension). Nowadays, his kidneys begin to suffer also. He has repeatedly rejected drinking water and said the cause is that the water is “impure”, “salty” and “not tasteless as it must be”. I have again and again asked him to stay hydrated, and mentioned to him how his health is declining due to such behaviors, but it barely worked!! I could force him to obey my orders, but you totally know how doing so is immoral ! Lying is also prohibited to me; I am seeking a “gentle”, “honest” and “satisfactory” manner to help him take care of himself and maintain a better health. Please help me; the water given to my dad is totally pure. I do not know what to do. What do you suggest?