I'm not a vet or a doctor but my vibes based science has me wondering if he's got some kinda liver/gallbladder issue, with the vomiting (you said he vomits acid) and runny poops, maybe if the food you feed him is high in fat you could try something higher in protein?
the internet says gallbladder issues in cats can present with jaundice so def look out for yellowing around his eyes and face
but idk again, not a vet, not a doctor, just a guy who dropped out of college once