Just to provide an update, after his latest vet visit the vet determined that the stool culture showed a form of bacteria known to be resistant to regular antibiotics. He set us up with a 2 part antibiotic to be fed 2x a day, and we started giving him that medicine in combination with the vitamin feedings. After a couple bad days, including one where my boy outright threw up some form of yellow chunky garbage, he has shown massive improvement!
He is now eating a lot more, poop is looking better, he is more social and more active. He still has another week to go on medicine but he looks and feels a lot more solid and stronger, and more alert. I really think the next vet visit will finally have some good news. i will provide an update when i get one!