Has anybody had Hyponatremia? Or know something about this condition?

Hyponatremia occurs when the concentration of sodium in your blood is abnormally low. It can be dangerous if your salt level gets TOO low, so please don't let it. If you're drinking just tons of water, you may not be needing to. You can drink something which contains electrolytes, or maybe just put a little pink Himalayan sea salt in your beverages. That should, at leas,t help.

Hyponatremia...low sodium in the blood. Can affect different things in the body, most notably, the heart.

https://www.webmd.com/a-to-z-guides/what-is-hypothermia

I'd never heard of it - just did a little quick reading and one thing I saw is in the elderly, who sometimes have poor appetite. it's called the 'tea and toast' syndrome - at least partially due a diet deficient in protein and vitamins. What does his doctor say?

Yeah, saw it allot in the Army. Experience it myself once.

What are your issues, I may have some advice.

Could be a side effects of medication(s)?

Yes when I was anorexic.