electrolyte imbalance , hyponatriemia

my father is 74 and suffering from cardiac and diabetic problem with specially cervical nerve compression on c3-c4, 5-6 6-7 also.
Last auguest28, 2010 I have checked his sodium and potasium level and I found that is hyponatrimic and his na+ level is on 116. I have tried all suggestion from here dr in india as it on my place. But still his na+ level is not coming at normal level.
Presently his na level is 123 mmol/l.
some more deatils:
weight 100 kg
obese
liver and kidney function test is right.
cbc reports are normal.
urine out put nearly 4 liter
intake fluid : nearly 4 liter
spot urine sodium was last month: 35mmol/l
What should I do in this position.
If you know some solution please tell me.
I will thankful for it.
thanks