A court date scheduled after a release date is unusual enough that it warrants a closer look at what that court appearance is actually for.
If the April court date is connected to the same case and charge he is currently serving time on, there may be a clerical or scheduling issue worth having an attorney look into. A court date that falls after the release date on the same matter does not typically make procedural sense, and getting clarity on what that hearing is for is important before assuming everything is fine.
The more likely explanation is that the April court date involves a separate pending matter entirely. A different charge, a civil case, a restitution hearing, or something that predates or runs parallel to his current incarceration. If that is the case, it may or may not affect his March release depending on what the charge is and whether there is a hold associated with it.
The critical question is whether any detainer or hold has been placed on him in connection with that April court date. A detainer means another jurisdiction has a claim on him and intends to take custody when his current sentence ends. If a detainer is in place, he does not walk out in March regardless of what the release date says. He transfers to the custody of whatever jurisdiction filed the detainer.
His attorney or case manager can confirm whether any detainers are attached to his record right now. That is the single most important piece of information to nail down before March arrives.