good that he's getting checked out then. The vet can advise you on the best ways to get weight on him.
Your breeder hasnt completed all vaccinations? .. this is usually done before homing kittens.
Are you having to pay for the vaccination he hasnt had yet?
this cost shouldnt have been passed onto you either if thats the case.
Only answer questions you are comfortable with, but now Im curious if your breeder has registered this kitten (with TICA/GCCF) and if you have a contract, and if they have a breeder website and if that site states that queens and studs are screened for illnesses inc HCM.
(if you share information on this be sure you dont name your breeder here in this forum)
Im dubious about a breeder selling a kitten that cries for food AND is underweight ... sounds a bit like cutting corners on the food budget to the detriment of the cat/s.
Avoiding vaccination costs (if thats the case) is most definitely cutting corners to make more money from sales and a breeder-member of this forum can back me up on that.
Im sorry to hear your new boy has a poorly tum.
It is possible he has a bug.. meaning both cats might now, you are right.
I hope not.
A tonne of people on this forum have experienced TF and the other gut infections and can offer you support and advice if that turns out to be the case.
Tummy wobbles due to re-homing stress is indeed common but unfortunately it can sometimes double as a convenient excuse for the breeder to explain diarrhoea.
You got him on the same food the breeder fed? (but correct amount now of course)
what was his very first poo in your home like?
how smelly is his poo?
Is your other cat eating his food and does he/she have a sensitive tum to new things?
Usual advice for runny tums is to switch to stomach calming foods like poached white fish and chicken but in this case you are dealing with a malnourished baby so I dont know if thats appropriate advice to pass along.
There are members of this forum that are breeders themselves and others that have a medical background, they are better placed to offer you advice to be honest.
Be sure to have him tested for bugs at or soon after (if still collecting samples) his first visit to your vet. If anything is discovered you must insist that the breeder covers your costs for treatment, this is part of the reason why breeder contracts (you have?) stipulate a purchased kitten be seen soon after being homed. Personally I would be adamant that the breeder pay to get the kitten treated (at YOUR vet not theirs, you will have receipts to prove costs) because every time a new customer fails to make a bad breeder pay for bugs caught from their cattery those breeders cash in and get away with cutting corners for profit.
If you other cat requires treatment also that will have to fall on your wallet alone I think.
I can tell from your posts that you have done or are doing research and you are 100% on the case.
its good to see.
Brrups to both your kittys... oh and Merry Xmas too