My son also was breastfed I would pump and add his salt to that in a bottle we did trial in error until he seemed ok with it we started with 2 ounces of milk with the salt and ended up with 3 ounces of milk with the salt.  Then I would breastfeed the rest of the feeding.  We did this until he was 6 months old.  We also divided the salt into six feedings to make the dose in each bottle less.