Dear bradda,
Welcome !:hello: . About the low sodium , while it has been reported , it is a pretty rare side effect of epilim. it is important for you to know that keppra can ALSO cause low sodium levels and you should titrate the drug accordingly with your doctor.
A persistent sodium level of 120 is not acceptable (normal - 135-145meq/L) and i feel perhaps a stay in the hospital for a day or two to correct this may be warranted. They will need to load him up with i'v saline slowly. if you don't want to go to the hospital , then i suggest , as skillefer said , a sport drink , or if not , ORS ( the salts packet to be dissolved in a litre of water that your dc gives you when you have diarrhoea.)
Again , i reiterate that you need to get this sodium thing sorted out asap as low sodium levels can push a patient with epilepsy into status epilepticus. And no , the body will not increase his potassium as well , so you don't have to worry about that. This is assuming that his kidneys are fine. For a few days, try to avoid gving him tap water, and quench his thirst with gatorade or the ORS solution , and restrict his pure water intake to 3-4 glasses a day max. You can also try mixing lemonade ( sugar , salt ,water, lemons) provided he's not diabetic. Try these measures out for a few days and get someone to repeat the electrolyte counts. See if they've gone up. Any idea if he is hypertensive ? if he is , then you may need a consult with your physician to determine if his BP or sodium levels should take precedence.
If you want more information , you can read up on hyponatremia on emedicine at the link given below:
http://www.emedicine.com/emerg/topic275.htm
It's written for doctors , but the language is pretty comprehensible. If you have any further questoins , please don't hesitate to ask