Hello, I hope I can help some with your issue.
To change a behavior like this, the first attempt is to ignore the behavior. Don't give the behavior attention. I would also suggest to him alternate behaviors.
Have him feel his belly to see if it is tight or have him go and sit on the toliet if he thinks he may need to go and reward that behavior.
To correct the behavior you must set-up a routine. Have him use the restroom at specific times and don't give him the option of needing to check. Once he has a set routine, you can relax that routine. If you would like to engage this conversation further or if I can help in another way. I look forward to responding.
Another idea would be to embed the desired behavior in a larger game. He gets a star on his behavior chart when he uses good hygene throughout the day. That includes washing hands before meals and after bathroom. Using his finger to check is bad hygene and he doesn't get the star.