Your vet is very bad not labelling meds, they should be labelled and dated, please make sure they do this.
There seems to be so much of this upset tummy around and have you tried restricting his diet to boiled chicken for a few days to see if that helps?
Lupin who is a one year old kitten was like this from the time he arrived with me two months ago, and being older probably not as serious as for a young kitten. It has taken since then until two weeks ago for his tummy to settle, chicken didnt help, white fish made him even worse and then the vet gave me some sensitive food in tryas that he ate for a couple of days but then wouldnt touch!
He is currently still on RC Gastro intestinal pouches along with a bit of food he steals from another cat of mine. I dont know if these would be appropriate for your kitten but its worth asking your vet. It took a few days, maybe a week for Lupin to start improving and he gradually got better and better, eventually being solid.
He was going to have tests cos we also went down the abs route but was a 2 week convenia jab and had he not improved that was the next step. It didnt pass to Ducha who he shares trays with so not infectious.
Sounds like your kitten has inherited this from his mother and by still feeding this may be the cause of why he is still ill.
I know very little about kittens and their mothers but isnt it time he stopped feeding? Angie will know about this.
When is the mother going to be neutered, cos think this will stop the feeding.
Hope you can get this sorted out