The video don't really show what I'm trying to describe. His belly changes size throughout the day depending on when he eats, but it's usually much larger than it seems like it should be for his body.  
When we first got him, he even walked a little awkwardly because he had to move his legs around his big belly. His stomach isn't hard or painful when I touch it, but it does feel bloated, almost like a balloon while still being soft and squishy.  
When I took him in for an exam and vaccines, they recommended a dewormer. I purchased the topical treatment from them and applied it to the back of his neck. It's been a little over a week now, and his belly has definitely gotten smaller, but it still seems larger than normal and looks somewhat inflated. His stool is also very foul-smelling and partially liquid. He also has black crust around his nose occasionally and after he eats, he often sounds congested, but I’m not sure if this is just because he is constantly purring.  
Should I be concerned about this or take him to the vet right away? Otherwise, he seems like a very healthy kitten. He has a great appetite, is very playful, and sleeps well at night. He's about 3 months old. He hasn't received his vaccines yet, but he was tested for feline leukemia and other common feline diseases, and everything came back negative.  
Has anyone experienced something similar or have any idea what could be causing this? Any advice would be greatly appreciated.

My kitten has a HUGE belly, why? And what do I do?