M.,
Poor baby! My little one (well not so little now, he's five years old) got a cast on his leg when he was 11 months old. A Toddler's Fracture is what they called it. I felt awful because I had fallen down the stairs while carrying him and so his little leg ended up in between my back and the stairs.
Anyhow, he had a cast for a month and we just had to make sure we kept it dry as well. I would wrap it in plastic when we gave him a bath or during feeding, but all the precautions I took, life happens and the cast got dirty and gross. By the end, when they took the cast off, his leg smelled terrible. Only after repeated bathing did the smell go away, but it was after a week. During the time I would put powder down the opening and clean it with cotton swabs of alcohol. I think it's inevitable for the skin underneath it to be the way it was because of it being enclosed for so long so just prepare yourself for the arm to be somewhat flaky and, obviously, lighter in color and a bit smaller in size.
Good luck!