I think your goat may have Caprine Arthritis Encephalitis or (CAE).

Symptoms- area of joint is severely swollen
- goat starts falling,then the pain gets so
bad that the goat just lies down
- to test for CAE you can bring your goat
to your vet and have his blood tested

There is no treatment,but pain can be lessened by applying salves that increases circulation.Warmth also helps to decrease the pain.I guess you give him warm bran mash,the stuff you give sick horses.If you don't know how to make it you can ask anyone who really knows horses or you can ask a feed or tack shop owner.

I really hope you findout what's wrong with your goat and I hope my comment made a difference.