The six months he served in county from August 2017 counts toward the 30-month sentence and should be credited against the total. That credit does not disappear because he was in county rather than a state or federal facility. Every day in custody from the date of arrest applies.
Here is the math. A 30-month sentence at 85% works out to 25.5 months. Subtract the six months already served in county and he has about 19.5 months remaining from the point he transferred to prison. Working forward from when he arrived at the prison facility, that calculation puts release significantly earlier than February 2020 if the six months county credit is properly applied.
The February 2020 date showing on the release listing may not have factored in the county time credit yet, or it may reflect the full sentence without the 85% good time reduction applied. Release date calculations on public databases are not always current or accurate, particularly early in a sentence before the facility has fully processed all the credit documentation.
The most reliable number comes from the facility itself. Have your boyfriend ask his case manager for a current time computation sheet that shows exactly how the county credit and good time have been applied. That document gives you the real projected release date rather than a figure that may not reflect all the credits he is entitled to.
If the county credit has not been applied correctly, that is something an attorney can address through the appropriate channels. Getting it right now rather than later ensures he is not sitting longer than necessary.