Unable to eat and vomiting tiredness.

My dad is 60 year old. From past one week he's not able to eat. He says if he sees food, he feels like vomiting and not eating anything. He drinks water but vomits that as well. For some days he ate liquid food and feeling better. He got his blood pressure checked which is normal. No diabetes, no jaundice and checked for viral diseases like dengue and all came fine. No fever. Apart from tiredness for which he got admitted and got drips, feeling like vomiting after seeing food, he is not complaining anything. No pain in the back side. No headache nothing. We are confused on what it could be. Worries if it's a serious illness underneath or just a simple general health issue. May I get some help from any doctors here and what would you think about this? Thank you