A ten year do five sentence means the judge has already determined that five years is the minimum time that must be served before parole eligibility kicks in. With a transfer date of January 21, 2016, that puts his earliest possible parole consideration around January 2021 under a strict reading of the sentence, not May 2017.
That said, the actual timing of a parole hearing is not set by a formula alone. The Department of Corrections, facility supervisors, and the Parole Board all have a hand in when a hearing gets scheduled. In some states, inmates can be considered for parole before the midpoint of the do time if their record is strong enough and the board is willing to hear the case early. Whether that applies in his state and under his specific sentence structure is something his attorney or case manager needs to clarify directly.
What is certain is that his behavior between now and any hearing date is everything. A clean disciplinary record, completion of recommended programming, participation in education or vocational work, and demonstrated accountability all factor into whether the board is willing to grant parole at the earliest opportunity or push it further down the road.
The full time of September 2020 represents the back end if parole is denied repeatedly or if his record does not support early release. That outcome is avoidable if he is intentional about how he conducts himself from this point forward.
Have him talk to his case manager about the specific parole eligibility date under his sentence structure. That number is the real starting point for any planning.