Five years after his abduction, she says their reunion is a dream come true.
"I haven't really celebrated Christmas or his birthday or anything. I was just waiting to get him back home. This will be our first Christmas together," Masumoto says.
According to the San Diego District Attorney's office, the boy had been living with his grandparents near Mexico City after his father, 37-year-old Julio Rocha kidnapped him and left him there.
Masumoto says she had just been given full custody of their son when Rocha took the boy to Mexico.
Last week her prayers were answered.
That's when authorities got a call from the grandparent's next door neighbor who came across a missing children's poster online with the boy's photo and recognized him.
After years of anguish, mother and child were finally reunited.
"As soon as he came. I just broke down in tears," recalls Masumoto.
But they are tears she says she would not shed if she ever has to face Keoni's father again.
"I think my first initial reaction would be to just slap him across the face. I don't think he cared about Keoni, it's just more the fact of taking him to hurt me and he accomplished that," she says.
Although she may have lost years of memories with her son, it only took one look at her little boy to find that love again.
"It was like, will I recognize my son? Will he recognize me? And when I saw him, that was it," she says.
Right now, Masumoto is focused on getting her son settled in back home.
Keoni is autistic, so she is trying to find him a school that specializes in special needs students.
Authorities are still looking for Rocha.
He is facing felony kidnapping charges.
Investigators say he might be going under the name Miguel Martinez and may be staying with relatives in Virginia or North Carolina.