You know, if you can get your LO to latch on and start feeding him straight from the tap in the evenings and on weekends then after a year you could stop pumping. Then he would get enough milk when you nursed.
Although I am saying this without knowing anything about his kidney problems though. But I know that after one many moms stop pumping and give their LOs water while they are at work and then just nurse when you will be together.
What a great stash you built up and I'm so sorry that you're finding you can't use it right now! I like the idea another pp gave you to mix it into smoothies when he's older.