It sounds like you better get Henry to the veterinarian before he gets worse. Runny stool is abnormal. Have your veterinarian perform a
fecalFecal culture
Fecal occult blood test
Fecal occult blood test (fobt)
Fecal smear
Flushable reagent stool blood test
Stool guaiac test and
giardiaGiardiasis test on his feces. Your veterinarian should treat him for a general parasite infection with strongid or drontal or equivalent whether he has parasites in his stool or not, since parasite shed intermittently, and cannot always be detected. Your veterinarian should also examine him, take his temperature and do blood tests for feline leukemia and feline aids as well as a CBC and Chemistry. I do not know why your first kitten died and I am very sorry for your loss but if you would like to prevent the same from happening to Henry it is better to be a bit proactive with his care.
If your female diet of a congenital or hereditary disease than Henry is safe; however, there are multiple causes for kittens not thriving and ultimately dying at a young age such as: fading kitten syndrome, feline leukemia, feline aids, other bacterial or viral diseases, congenital or hereditary disorders, parasites, toxins, hypoglycemia, trauma, and a multitude of other causes. Your veterinarian would not be able to detect many of these on a routine first office visit unless he did the blood work and other tests listed above. Most of the time blood tests are not necessary for young kittens at a first healthy office visit, and I believe that your kitten was healthy on that first visit. Blisters on the tongue could mean that she could have gotten into something toxic, or poisonous. I do not know what purple around the eyes would mean. You would have to give me more information about that. The hypoxia (lack of oxygen) could be caused by an anaphylaxis reaction secondary to a toxin, or deadly insect bite. I would make sure there are no toxins available around your property that the kittens could have gotten into and remove it from Henry’s reach, just in case.
If you would really like to find out the cause of your female kitten’s death you would have to have a post-mortem (autopsy) performed with microscopic examination of multiple organs by a pathologist.
Sorry again about your loss and I hope all goes well with Henry!