2yo cat, kidney cysts (likely inherited) - heat as trigger? + food/experience tips wanted

My male cat is 2 years old. About 2 weeks ago I noticed something was off with his ribcage — bumps on each side. I took him to the vet, and 2 days ago she diagnosed kidney cysts. His kidneys are swollen. She wanted to do a blood test to check for kidney failure, but I decided against it for now — I was too stressed and shocked in that moment to make a decision I'd feel good about (I have autism, and sudden medical stress like this hits me hard). I may revisit that decision once I've had more time.

I believe this is likely inherited from one of his parents, since it doesn't seem to line up with anything else in his life or environment.

A few things I'm hoping to hear about from people who've gone through this with their own cat:

1. Heat as a trigger? It's been an intense heatwave here (since good 3 months barely any rain), and my gut feeling is that the heat put extra stress on his kidneys and made this become visible/symptomatic now. Has anyone else noticed a heat connection?

2. Food and weight gain. He won't touch the prescription kidney food (duck flavor) or the food I give my other cats, but he eats a low-protein chicken paste really well. Not sure if it's the duck flavor he dislikes or something else. I'd love tips on getting some weight back on him and keeping the swelling down as much as possible.

3. How did it go for your cat? Better, worse, stable? I'd really value hearing real stories, even hard ones, from people who've actually lived this.

A bit about him so you know who I'm asking for: he has the most unique personality — total cuddle bug, gives head bumps constantly, but hisses and growls if you hold him too long (very much on his own terms). We still go on our late evening walk together and he still wants to, even though I can tell he tires a little faster now. His coat still looks good and he's still very much himself.

He's my soul cat and I'm not ready to lose him. Any experience, tips, or hacks are welcome.