Cyclic vomiting leads to low sodium leve

My father has been admitted to hospital from last 5 consecutive years due to cyclic vomiting ,weakness . & confusion  Each  and every reports are normal except sodium level. Sodium levels drops 125 .We have visited several big hospitals after treatment of low sodium level his condition improves but after 6 month or one year same symptoms occurs, severe vomiting, weakness & seizure and needs to be hospitalised. No doctor has diagnosed clear cut reason for cyclic vomiting leads to low sodium level as his all reports are normal. My father weight is dropping continuously since 5 years three has lost 10kg weight his age is 71 years.kindly pls suggest for prevention of cycling vomiting leads to low sodium level.