Officials say Briant Rodriguez is fatigued but otherwise in good physical condition.
The youngster underwent physical and emotional evaluations at a local hospital after returning from the Mexican border town of Mexicali, where he was found wandering the streets.
Briant was kidnapped by two armed men who snatched him from his house after bursting in and tying up his family, San Bernardino County Sheriff Rod Hoops said.
"I've been doing this for 30 years. I'm not saying it doesn't happen ... but the odds of finding him safe and alive - the odds of finding him alive - went down every day," Hoops said. "Our detectives never gave up."
Briant and his mother, Maria Rosalina Millan, were reunited earlier Saturday in Calexico, Calif., a day after detectives and FBI agents received a call from officials in the neighboring Mexicali saying they found a boy that could be Briant, sheriff's Lt. Rick Ells said.
Ells said a municipal police officer in Mexicali found the boy wandering the streets by himself late Thursday.
The officer initially thought the child was from the neighborhood and took him to several houses seeking his family before leaving him in child protective custody, Ells said.
Mexican authorities did not realize who the child was and didn't notify U.S. authorities until Friday afternoon, he said.
Ells said Briant's mother and investigators boarded a plane to Calexico Friday night.
FBI agents crossed the border and handled immigration paperwork for the boy, who holds dual U.S. and Mexican citizenship, he said.
The agents brought Briant back across the border for the reunion Saturday, Hoops said.
"I talked with one of the FBI agents... that came across the border and watched the mother and the son reunited and he said the last thing he saw was Briant clutching his mother's neck," Hoops said, becoming visibly choked up. "And that kind of says it all."
Hoops said Millan was not reunited with Briant until the day after her arrival in Calexico because of the amount of paperwork involved in the cross-border incident.
Gerardo Franco, a spokesman for the Baja California state prosecutor's office, said Millan also was not immediately able to see the boy because he understood she was an illegal immigrant in the U.S. and apparently did not want to risk crossing the border.
Hoops declined to comment on Millan's immigration status, but stressed that Briant is an American citizen.
"He was born a hospital in San Bernardino and he has every right to be here," Hoops said. "That's why we worked so hard to get him back."
Hoops did not know how long Briant had been left alone in Mexicali, but said he appeared unharmed and was in good spirits. His long curly locks had been shaved off.
He said authorities believed they were closing in on at least two men suspected in the abduction when the boy was found.
"We identified some suspects," he said. "I can't specifically say if the suspects let him go or if he wandered away, but I have to think the suspects let him go."
Federal and local investigators have said they were looking at several theories, including that the Spanish-speaking kidnappers were from Mexico and may have had ties to organized crime there.
Millan said two men burst into her modest, single-story home about 60 miles east of Los Angeles on May 3, tied up the family, stole money and other property then left with her youngest child.
The kidnappers had not demanded a ransom and the initial investigation pointed to the kidnappers being strangers to the family.
Hoops said the investigators were not looking in Mexicali when Briant was found.
"We were searching leads all over California and all over the country," "I don't want to say we got lucky because our investigators, they worked their tails off."
By Associated Press Writer Gillian Flaccus; AP writers Daisy Nguyen in Los Angeles and Mariana Martinez and Alexandra Olson in Mexico City contributed to this report