I think that kitty will be okay. Diarrhea is really common in young cats - I have fostered some that had diarrhea for weeks and nothing was wrong with them. They either grew out of it or I gave them some medicine to help with it until they were older.
FIP is really scary - kittens usually catch it from their mothers while in the womb or during birth itself. Do you know about the mother and the rest of his siblings? If they are doing fine, then it probably is not FIP. I once fostered a kitten who ended up having FIP. His mother and brothers were all dead and some one found him alone nearby when he was 4 weeks old. Since there is no way to really test for FIP, I kept him and looked out for symptoms. His stomach was a little bloated (he had worms as well that were treated asap), but the rest of him was really boney. He ate fine, but was never active and had very little energy. Two weeks later he was walking in circles and running into objects - the neurological symptoms. I took him in already knowing he had in fact developed FIP. The vet felt his stomach and chest and immediately felt the fluid buildup.
Your kitty doesn't sound like the one I had - if he has energy and is growing, that would be strange if he had FIP. If you are concerned, another trip to the vet and a quick exam would be able to tell you if the hard stomach was FIP or not. Kittens can sometimes have that big belly from overeating, too.
I like to go by the idea that if a kitty is sick, he will let you know. If a kitty is growing and thriving and playing and eating and pooping, he has a good chance of being as healthy as he seems.