My uncle is not eating food unless we force him. He is getting medical treatment but we are concern about his overall condition and mental health due to low levels of sodium. Are there supplement food available that can be given to maintain-his sodium level?
As you can see, too much liquid can cause this problem.
You really need to have a doctor guide you on this.
There would be a way different reply from me depending on why he is not eating. If he is not eating because he is at End of Life I would say you should not "force" him to eat.
If he is not eating due to anorexia that would be different, although you still should not "force" someone to eat.
If he is not eating due to a medical diagnosis that makes food unappetizing, or medication makes him nauseous that is a different problem.
Talk to his doctor, ask for a consultation with a dietitian for some ideas.
Sports drinks have a lot of sodium.
A lot of fermented foods sauerkraut and pickles for example contain high amounts of sodium
Most processed foods have a pretty high sodium content.
There are pills that he could take that would help retain / add sodium to his body.
Get him to a doctor.