I seriously doubt he has FIP. That is the thing that causes fluid accumulation in the chest cavity and abdomen. He would have difficulty breathing most likely with this. He would have a lot of other symptoms if this were the case: Weight loss, not eating, trouble breathing, fever, lethargy, and many more. He wouldn't be happy, playful, eating, drinking, and acting like a normal kitten. So don't worry about FIP.
I don't know what worming medication you used, but in the future, use Profender. This treats all worms. Plus it is an easy spot on treatment between the shoulder blades like Frontline. Much easier than pills. Since he has been wormed twice already and you don't see any in his stool or rectum, he is probably worm free by now.
Kittens gain all their weight in their belly. Like I said my kitten looks just like this too. She has a big round belly, sides stick out everything. She was skinny little legs, and even a bony back. She is totally healthy and normal, and ready for her spay next week.
He'll thin out in the belly soon, and start to gain weight evenly through out his body.