Unfortunately 2-3 weeks is going to yield little to no drastic improvements, but we appreciate the update!! :)
Were you able to get him a salt block and SandClear?
As soon as the vet comes out, I'd ask them about testing a fecal sample. If your vet cannot provide a fecal test for Switch, SmartPak has a kit that you can purchase online (here), but it doesn't test for pinworms or tapeworms. A fecal test will help tell you which dewormer you need to use, and which parasites you're targeting. You could go the old fashioned way and use a rotational deworming schedule, but that method is proving itself a bit outdated, and not always effective. While I'm almost certain that he does have parasites, it's a worthy investment. After you're able eradicate the parasites, it's very likely that you'll find you only have to deworm twice a year (depending on your individual horse). I know a vet whose horses only needed to be dewormed once last year, according to a fecal test!
I'm so glad to see the tiny bit of improvement that we can see! :) Keep us updated! :)