August Cartwright had presumably killed him and taken his place around 2014.
Campbell worked as a plastic surgeon in Gotham City since 2000 becoming extremely well-known and respected for both his liposuction philanthropic work. He was also one of the most renowned teachers at Gotham University.
At some point in his life he married.
Around 2014, Campbell took a sabbatical year. Around the same time August Cartwright mysteriously disappeared and was believed to be dead, however he actually resurfaced in Gotham City as Ethan Campbell, likely having killed him and taken his place.