Best Gastrointestinal doctors?

Hi I’m looking for a gastroenterologist(or stomach focused) doctor for my father who struggles with issues related to this area. We’ve tried all the doctors here in east tx, and they always tell him the same thing- either prescribe pills that end up not working or that there’s nothing identifiable. I want to find him someone new and more effective, any recommendations here in Houston or anywhere else in Texas? Thank you!