Winter damper setting for humidifiers
The damper should be wide open since the wall or duct mounted humidistat (that should have been installed) controls the operation of the humidifier and how much humidity is introduced into the living space. You are right not to agree with your brother since he is not correct. If you have A/C then you close the damper in the summer, otherwise leave it open.
Don't forget that most drum type humidifiers need cleaned at least once a year. I have a drum humidifier and soak the media pad in a bucket filled with vinegar and clean the reservoir pan as well. My humidifier also has a drain to flush the reservoir numerous times during the winter.