Runty Kitten Mystery- Any theories?

I foster litters of kittens with mama cats for my local animal shelter. My current litter is currently just over 6 weeks old. This litter has had a rough time since the beginning. The mama cat is very very thin, and had a large litter (6 kittens). All 6 kittens were very small at birth- between 2.3 and 2.7 oz each. One kitten died in the first week of unknown causes, or fading kitten syndrome. Four of the kittens have developed normally and are healthy, normal 6 week olds. The 5th kitten is runty and TINY for his age. I brought him to the vet a little over 2 weeks ago- he had an umbilical infection. The vet prescibed Clavamox and the infection cleared up right away. We both hoped that his growth rate would pick up, but it hasn't. He currently weighs about 8 oz. We went back to the vet yesterday, hoping to get some answers. I was mainly concerned about his belly- its huge in proportion to the rest of his body, and about his overall stunted growth. I was afraid it might be FIP. The vet looked him over pretty well- she said that his belly is concerning, but that he doesn't have any other symptoms that would indicate FIP. He is too tiny to have blood work done, and at this point an x-ray won't show much. She said she is leaning more toward some type of congenital abnormality, maybe something involving the liver or heart. However, she did not detect a heart murmur. Has anyone experienced something like this? Any ideas?