More info from Japan-Zone.com

"Singer Amuro Namie (27) has won parental rights to her 7-year old son. She divorced TRF dancer Sam (43) in July 2002 after five years of marriage, at which time Sam was given "parental authority" over their son, while Amuro had the "upbringing authority." Following an arbitration by the family court, Amuro now has both. Both Amuro and TRF are with the Avex record label and are said to remain friends."

I had actually heard that Namie got full custody of her child a while ago but I guess now it's officially on paper.