When my son was 1st diagnosed he had an NG tube which we used to give him his sodium suppliment.  The endo told us we needed to transition to giving my sons his supplementation via mouth and we told the doc that he would spit it up or refuse to eat (if we mixed the dose with formula).  The doc then told us to simply use non-iodized table salt (he did some math to determine the proper dosing amout) and mix it into my son’s formula when we make it.  So for about 2 months now we make 20oz of formula and mix in 1/2 teaspoon of table salt and my son’s sodium (we get it checked 1-2 times a month) is always between 138 and 140.  We have found this to be a life saver!  My son typically eats between 25-32 oz of formula a day. 

It is worth asking your endo........