Oopsie, I didn't see his age before - 40! Bless His Precious Heart!
If his teeth have been checked, fecals taken to the vet to check worm load, it is very possible he either has endocrine issues that come along with Equine Metabolic Syndrome (which is not
the same as insulin resistance), or he may have early cushings.
EMS can cause an easy keeper (which ponies are) to drop weight.
Because he is 40 or thereabouts, I might ask the vet about my suggestion first:
If you belong to Sam's Club or some other such shopping place:
Buy some chastetree berries and run them thru the blender. I buy mine from an equine herbal place but it's cheaper if you have a Sam's Club nearby (I don't).
Mine looks like coarse ground pepper and I feed 1/8 house hold measuring cup, not quite to the top for my 975 pound horse.
I'm lousy at math, so you figure out the difference for your pony - lol lol
Or I think you might be able to buy pure chastetree or chasteberry (also called Vitex) at certain health food stores; just make sure there's nothing else in it.
My point is, the chastetree should help him quickly finish shedding if he has any sort of metabolic problems. If that is the case, he needs his diet re-vamped. No grains whatsoever, zip zero, nadda
He's a cutie-patootie