Sounds like he needs a different vet. I would be calling someone else if a colt-then-gelding had chronic diarrhea. Horses that have chronic diarrhea can get dehydrated or not absorb nutrients they need to grow and mature normally. And they may be getting infected (even repeatedly, if you have other horses) with an illness or parasite. Your other horses could be immune to something that makes him sick.
Almost anything could cause diarrhea in a young gelding.
It could be that a product you're feeding, that he has a food allergy to that product.
It could be that your pasture or hay has a plant in it that bothers him. He may be picking up a weed that tastes good but causes diarrhea.
You may have wormed him recently, but the wormer you used may not have been in a sufficient dose to kill the parasites he has.
Or the wormer you used may not have killed the type of parasite he has.
The wormer could have lost its effectiveness (check label expire date).
Or the youngster could have gotten reinfected with worms from the other horses or the ground, after he was wormed.
He could have a digestive disorder, such as a malapsorption type of disorder.
He could have ulcers. Some young horses get ulcers. A fairly large percentage of foals have ulcers, sometimes they don't clear up as the horse grows.
He could have a chronic infection, or a tumor or a congenital abnormality of the gut.
So vet.
A good place to start is to collect a fresh fecal sample, and have it analyzed by your veterinary clinic. Have a blood sample taken, and have the blood tested for infection, anemia, a regular panel.
If he were mine, I'd first take him off pasture and all supplements, treats and bagged feeds, an just feed hay for 3 weeks and see if that makes a difference.
I'd clean out my water troughs and buckets and feed tubs, and look for any possibility he could be nibbling on something or getting into something in the barn or around the yard - chemicals, household cleaners, car products, varnishes, etc. I'd lock up all feeds and be sure horses could not get into them and do 'self serve'.
I'd walk my pasture and see what plants were growing there - if I could not identify some I'd pick a sample and take them to my ag extension office of my county. I'd pull all weeds and anything that the ag office thought would be a problem. I'd get a list of undesirable trees from them. I'd look for cherry tree leaves, black walnut trees, etc, and remove those trees from near or in the pasture - even if they aren't dropping leaves on the pasture - some are poisonous through the ground. I'd look for any buckets or trash that might have been dumped in an unnoticed area of my pasture. I'd look to be sure neighbors were not dumping their trimmings or grass clippings into my pasture. I'd look to see if any of my neighbor's septic fields were leaking into my pasture after rains. I'd look at my fence and see if the horses were chewing ont he wood, and put no-chew on the wood or run a little lightweight hot electric string along the board.
Before I took the youngster out of the pasture, I'd try to just watch him for a day and see if he was getting into anything.
You can find odd things with horses. One of mine can't eat any fine grasses - gets diarrhea. Can't soak his hay - he gets diarrhea. Can't feed him clover, he gets diarrhea. So it can also be an individual thing.