From the pic posted of Rusty, he does not look that thin.. Can you feel his ribs under all that hair ? Winter hair can hide problems.He may need some extra calories in winter, beet pulp and maybe a senior feed for the vitamins . He may just be arthritic.
Bute is for pain. Glad to hear he is no longer limping. He looks to have a hay belly or worms, you can find generic ivermectin fairly cheap and strongid .
You can go 8 weeks on farrier work, if he has slow growing hooves. I have a couple of horses that go much longer, and they just don't grow hoof and keep them worn off ,
My horses are all barefoot.
Everyone who has horses was a Beginner at some point. Don't get discouraged by rude comments . There is only one dumb question , and that is the one that is not asked.