Terry Allen Nealy has confessed to the killing at the motel, which is about a mile from where Aaron Vu, 10, was shot at his parents' nail salon, the South Florida Sun-Sentinel reported Monday.
The boy's father, Hai Nam Vu, was wounded in the Nov. 22 armed robbery of Hong Kong Nails.
Aaron and his two younger siblings often spent time at the salon as their parents worked. Aaron and his father were injured when one of two armed robbers fired at them as they ran out the door, police say.
The father, 43, was hit in the shoulder. The bullet that fatally struck the boy hit an artery and caused massive blood loss, his family said.
Nealy, 22, was arrested early Sunday at his home in Miami Gardens, North Miami police Maj. Neal Cuevas said. He has been charged with first-degree murder in the death of Luis Perez during a motel robbery Nov. 1.
Anthawn Ragan Jr., 19, was arrested Nov. 26 and charged with murder in the deaths at the motel and salon. He also faces charges in another armed robbery and the battering of a police officer as he fled the scene.
Nealy and Ragan may have been partners in a string of crimes in the area, Cuevas said.