Final update: Mac died in my arms this morning... I'm heartbroken. I got up to let everyone out into the yard and Mac was fast asleep when I opened the cage. He didn't even wake up until I picked him up.
I put him outside briefly and watched him stand but not walk... he seemed like he was just exhausted, like all he wanted to do is sleep. His wings even dropped to the ground for a time and when I put him in his pool, he didn't swim.
I took him inside, wrapped him in a towel, and held him. We cuddled for a time and he suddenly woke up with a start and so I put him on the floor. He couldn't stand and when I tried to hold him upright, he had a brief seizure and collapsed in my hands. We brought in Pistachio and Cheese and they sniffed him for a little and pressed their heads on him before walking back outside.
We dug him a grave and buried him.
I'm sorry to bring this news but it feels good to type it out. Thank you to everyone for being there for us and helping me give him more time to live a happy and healthy life with his adopted siblings. He was so loved.