As U r saying ur father is having Sodium defficiency....The reason was not found clearly...any recent H/of Diarrhhoea/Dysentry/Vomittings?)..For time being let him take a glass of water with small spoonful of Salt&1 Table spoonful of
Sugar for few days...daily or thtee times/day as per his need.(But shouldnt b contd for l9ng,as excessive Sodium intake will do Harm again),&Parallally,Try Compensate that depletion by giving him all types of Fruits&Fruit juices/Curd/Butter milk/Milk&plenty of normal water.
If u continue this forever,He vl b healthy aswell.
TC.