They declared him dead based on a brainscan? There is a standard brain death test that it looks like they didn't perform (probably because it takes a bit of time). I bet that would have shown that he wasn't brain dead. Sure, taking someone off the ventilator for 8 minutes is nerve-wracking, but it needs to be done.

Yell at them, if they respond, not brain dead.

Pinch them hard in sensitive areas. If they move, not brain dead.

Stick a cotton swab up there nose (ouch), if they move, not brain dead.

Shine a light in there eyes. If they constrict, not braindead.

Turn their head suddenly, if their eyes act like dolls eyes and stay focused on the same point rather than move with the head, not brain-dead.

Pour ice cold water in each ear for a minute, if their eyes move, not brain-dead.

Save this one for last. Put a secondary tube of 100% O2 down their throats, take arterial blood gas to check blood CO2 levels, remove primary ventilator for 8 minutes (that forces breaths in and out, O2 tube just lets O2 in but doesn't force their chest to expand and retract) CO2 will build up in their bodies, which is a strong stimulus for the brain to make them breathe, watch and listen carefully for signs of breathing. After 8 minutes, check Arterial blood gas again. If the CO2 went up and they were not stimulated to breathe, then they are probably brain dead.