We have an 8-month-old daughter who I exclusively breastfed up to six months. I never mixed the sodium chloride with breastmilk, we always gave it to her (we get it in liquid form from the pharmacy) in an oral syringe. When we first started giving it to her, we had to make precautions to space the timing of doses away from nursing so that she wouldn’t associate the sodium with her food. Now that your baby seems to be beginning to make that association, maybe that would help? Mixing with water rather than milk might help remove the association as well.