The boy, who was the subject of an Amber Alert Thursday morning, was found in a downtown Toronto hotel nearly a day after he went missing in the Niagara Region.
He was with his mother, who was taken into custody, while the uncle was arrested in a separate location.
Police said late Thursday night that proceedings on abduction charges against the pair can't begin without the consent of the provincial Attorney General.
Police said the mother and uncle have been released from custody pending the outcome of the investigation.
Niagara Regional police spokesman Derek Watson says the boy and his father are U.S. citizens who live in Georgia and were in Niagara Falls for a family visit to Marineland.
The trip was arranged partly so the boy could meet his biological mother, Watson said.
The father and mother were estranged and the boy had not seen her in more than five years, he said.
Several family members gathered at the amusement park Wednesday and police allege the mother and uncle — who arrived together — split from the group around 3 p.m., taking the boy with them.
It wasn't until around 9 p.m., when the park was closed, that the others realized the boy was missing, Watson said.
He said the family went to the front entrance and saw the pair's vehicle was gone.
Police later obtained a video of the mother withdrawing money with her son at an ATM in Mississauga just after 9 p.m. that night.
On Thursday morning, provincial police pulled over the vehicle linked with the suspects in Toronto and arrested the uncle, who was on board.
That led them to the hotel where the boy and his mother were found, police said.
"The child's safe and fine and everything worked out well," said Toronto police Sgt. Andy Schneider.
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