Working through the math gives a reasonable estimate, though the exact date depends on the rules of the specific state and what his judgment and commitment order says.
The standard framework in most states is that inmates serve 85 percent of their sentence before becoming eligible for release. On a 24 month sentence, 85 percent works out to 20.4 months of total time to serve.
He has approximately 100 days in custody already, which converts to roughly 3.3 months. Subtracting that from the 20.4 month total leaves about 17.1 months of remaining time from the point of sentencing on September 19, 2018.
Adding 17.1 months to September 2018 puts the estimated release date around late June or July 2020.
The reason he keeps getting told different dates is likely a combination of factors. Good time credit calculations can shift if he picks up infractions or completes programs. The 85 percent rule is not universal and some states calculate differently. And the administrative process of computing release dates is not always consistent between departments or staff members at the facility.
The most authoritative number comes from two sources. First, the official state corrections department offender locator, which reflects the current calculated release date based on the actual record. Second, his case manager at the facility, who can pull the specific good time calculation and give him the current projected date based on his conduct to date.
Until he hears a consistent date from the official system, July 2020 is a reasonable working estimate.