Have they done an ultrasound and tested the kidney and liver function?
I would ask your vet (or another vet) to test the kidney, liver, and either do an ultrasound or radiographs to check for inflammation, blockages, and tumors. I would also ask the vet to run a full blood chemistry panel regardless of his young age. Are his gums pink (some cats have black splashes on their gums, find a pink section- there shouldn't be white gums)? Are his eyes white or are they yellow? When you press on his gums, how long does it take for the color to return (in seconds)?
When he's having an episode, mark the date, the time, and the length of time on a calendar. Keep a calendar, mark his litter box uses, his eating schedule (don't free-feed him, put him on a schedule and get his body regulated), note on the calendar when he vomits and how much and what color, note on the calendar what his bowel movements look like. Take all of this information to your vet, and ask if the vet thinks you need to go to an internist.
If it were my cat, I'd have a full blood chemistry done, it would check the liver and kidney function. Then I'd have xrays or ultrasound done and see an internist. Also I'd check him for FIP and other coronavirus indications (more bloodwork) since the coronavirus do affect the abdomen.
I hope that he feels better soon.