I'm so, so sorry :( I know how hard it is to have a sick pet, especially one that might have an incurable problem.
I wish you luck and I really hope you find out it's not FIP. It's such a terrible, painful disease. For both pet and owner.

Thank you so much for your kind words. I appreciate it.

What's wrong with him/her?

He had FIP. It's a fatal disease that mutates and attacks the whole body.