Sodium is a part of the urea and electrolytes or BUN in the US. It also has potassium and creatinine in it and is generally considered to be a measure of renal function.
Do you have any numbers?
Low sodium and high potassium can be indicative of Addison's disease, he would need to have his cortisol checked.
There is a bit of room for different levels in different people, so down 120-160 nmol/L is acceptable, but I would have it checked again.