The 2024 release date is likely already the adjusted figure reflecting the good time credit built into the sentence. Most state systems apply a standard good time reduction at the beginning of the sentence, typically around 15 percent, and that adjustment is what produces the out date you see rather than the raw third anniversary of his sentence start.
If the release date already reflects that credit, the question becomes whether anything changes it between now and 2024. The main way a release date moves later is through disciplinary infractions. Serious write-ups can result in good time being taken back, pushing the date out. As long as he stays clean on conduct, the date holds.
The main way it could move earlier is if there are any additional sentence reduction programs available at his facility, such as earned time credits for programming completion in states that offer those, or in rare cases facility overcrowding releases. But in most situations, the 2024 date is the realistic target.
The best thing he can do between now and then is keep his record clean, complete any available programming, and stay focused on what comes after.