17 and a Morgan is the Perfect Storm to be developing some form of metabolic issues, including cushings.
I really would have a vet draw blood to check insulin AND cortisol levels; that means two viles of blood. Plus the vet can give the horse an over-all physical that should include a quick brush of his hand down the coat to check hair texture.
The horse shouldn't have any grain whatsoever and if the horse is on pasture 24/7, get the grazing muzzle on for the daylight hours. It can come off at night, muzzles are not meant to be left on 24 hours a day.
Other things that can cause slow shedding and change the texture of the coat are:
I know from personal experience with my EMS horse who also has hind gut ulcers. Once the vet diagnosed him and I started treating him for the ulcers, he shed the rest of his coat in less than ten days.
I say "rest of his coat" because I had started clipping him, a little at a time. I'm an hour north of the Alabama border, so we experience early warmth and humidity, although not as bad as your area:(
I had given him a Tuxedo clip first. Two weeks later, I clipped his belly and half way up his barrel.
At that point he colicked really bad, twice in the same month, and that's when the vet diagnosed the hind gut ulcers.
Soon as he got on ulcer meds, he finished shedding on his own.
My point to all that is, in all likelihood, something wrong internally with your horse and the vet needs to get involved