I really don't think anything is wrong with him. I too have a kitten that is 10/11 weeks, and she has the same "buddha belly" too. Many kittens do at this age. One thing that can cause it, and I don't if you noticed any, is gas. Just like human babies, gas can be issue for kittens and pups too. The best way to deal with it is by giving a tsp a day of plain yogurt. This will help get rid of gas and stomach bloat.
But they do tend to have that round baby belly at this age, and its normal.
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.