It might have been better if he just didn't go in (assuming that's in accordance with his beliefs about the war). I haven't checked the current policy, that's what I was reading about 2 years ago, so I'd double check that before acting on it as if it's still true. A GI rights group can probably help you out with current info on that.
I wouldn't trust them to keep a person out for a history of cancer.
Here's a woman whose Army mammogram turned up a cancerous tumor in April, had a lumpectomy in May, finished radiation in June, and later in June was being shipped to Iraq. She had the option to stay back for further treatment, it wasn't forced on her, but if they are deploying people within a few weeks of radiation, declaring them to be cancer free, I don't think your brother's case is necessarily going to keep him home.
http://www.military.com/NewsContent/0,13319,140079,00.html