Members of the KATUSA are part of the Korean Army and are not considered part US military.
As such, the laws pertaining to US Armed Services members in regards to their cell phone service would likely not apply.
Your brother can suspend in the manner I noted above. He can suspend will out billing for a maximum of 180 days in a 12 month period. Or he can suspend with billing and keep paying during the two years.
And as I also noted above, you do all of this by calling Customer Service.