My daughter Loghyn (18 mths SWCAH) was on extra sodium too . . . we just dissolved 1 standard salt tablet in every 8 ozs of fluid she drank.  We stopped buying the salt tablets when she moved on to solid food, but we salt all of her food & still put about 1/8 tsp of table salt in whatever she’s drinking.  I agree with Carol that he may just need the supplementation. 

Another thing, we were told when Loghyn was real tiny that children’s kidneys don’t absorb the florinef as well as an adult, so it’s not near as effective as it is at later ages . . . that’s why our endo does the sodium replenishment as well.

Good luck!

Jess