Male kitten, ~4 months old, 4.9 lbs, neutered

We got him neutered 21 days ago. Since we brought him home his appetite has decreased, he is not pooping very much, and his abdomen has become very swollen. Aside from that he seems perfectly normal. I took him back to the vet this morning because I was concerned about his stomach. After a scan and some blood work they told me he has FIP and would need to be euthanized. My questions are: Is it possible the neutering had something to do with him contracting the illness? He is a rescue, could it have come from something he came in contact with before we got him? Is FIP rare? I've never heard of it until now? Do you have any advice for caring for him or knowing when the right time to euthanize would be?

Thanks for taking the time to read about my baby and for any advice.