Ben entered a rehabilitation facility in Malibu on Aug. 22, in order to receive addiction recovery treatment. The "Justice League" star has battled alcoholism for years and previously received treatment in 2001 and 2017. Ben has been dating Playboy model Shauna Sexton -- who's been visiting him in rehab -- following his split from "Saturday Night Live" producer Lindsay Shookus and divorce from Jennifer Garner.
Casey said that he thinks his brother is "very brave" for seeking help.
"He's recovering and I think it's very brave of him to take care of what he needs to take care of," the "Manchester by the Sea" star told E! News at the festival. "He's handling his business and getting better. I look forward to him being out and back on his feet."
Casey revealed last year that he has struggled with alcohol addiction in the past and is three years sober.
"It's just so many times, going to these meetings and sitting in circles, and talking about it all," he said. "So when it came time, when I realized that I had to stop, I felt like I had already put in all that time, so I just kind of white-knuckled it at home and imagined myself in a circle. It worked."