My Dad Is 90 Years Old, Has Liver Failure, Kidney

My dad is 90 years old, has liver failure, kidney failure and heart failure. He has been on Hospice since last July. He gets around fairly well, takes a shower on his own and takes a bus to the casino once or twice a week. Every month or two he goes through a spell when he does not want to eat. The thought of eating makes him nausea. If he forces himself to eat a little he gags and wants to vomit. His mouth stays dry and some foods he can not swallow. This may pass after a couple of weeks and he does o.k. eating again for a while. What can be causing this?