Since your baby seems to be taking the saline well I think I would stick with the current plan (my son hated the saline so much!)  Ask your pharmacist if you could store part of the saline solution in the freezer for a longer period of time.  You could transfer the solution into smaller bottles and then remove them from the freezer as needed. 

Hopefully your son won’t need the saline much longer.  My endocrinologist told me I could stop giving my son the saline after the first month or two.  I think different endocrinologists have varying opinions about the importance of salt supplements.