September is the realistic target, but approved and released are two different things and the gap between them requires some patience.
When the parole board approves release, that decision triggers an administrative process that takes time to work through. A parole plan has to be finalized and verified, supervision conditions have to be established, and paperwork has to move between the board, the facility, and the supervising parole office in the jurisdiction where he will be living. None of that happens overnight.
The September date on file is the board's target, and in most cases the actual release lands within that same month, sometimes earlier in the month if everything moves smoothly and sometimes toward the end if the paperwork takes longer than expected. Delays beyond the target month do happen, but they are less common when a specific date has already been established by the board.
The most useful things you can do between now and then are making sure his release plan is solid and confirmed on your end, that the address is verified and stable, and that you are reachable if the supervising parole officer needs to confirm anything. Any loose end in the release plan is a potential source of delay, and the cleaner everything looks on paper the smoother the actual release tends to go.
Stay in contact with his case manager as September approaches. A quick check-in a few weeks out can surface any issues while there is still time to address them.