Hey Bronwyn, I just saw this thread now! I can't add much about his short time with me, except that he was on aspen shavings for most of it. I put the carefresh in his cage the night before he went to you, as I didn't have any shavings left.
Unfortunately I didn't have a lot of time to spend with him, but he was fairly active in his cage - going up and down the ramp and levels, in and out of his house, and would come to the door to greet me when I was close by. I didn't see any porphyrin or notice any strange breathing.
I'm so sorry to hear that he is not doing well! That's very upsetting news, he's a great little dude. However, heart meds are definitely an option. I was thinking possible heart condition as I was reading through all your posts! He has classic signs, and I do believe heart medication could improve his quality of life and extend the length of it.
Please let me know if you want to investigate that option further and need help. Your dad can discuss with my vet if he would like a medical point of view, but there is also lots of info out there such as in the Rat Guide, and of course many of us have personal experience with it.
I hope your cat will be okay too! It's very tough when things happen all at once.