According to ET, Foster filed the motion on Tuesday which claimed Usher has "failed to personally exercise physical custody of the minor children."
Foster also claimed that Usher is rarely home and Monday's accident happened because the child was left unsupervised.
The former couple has two sons together -- Usher Raymond V, 5, and Naviyd Ely Raymond, 4.
While Usher was not home at the time, his aunt and maid quickly noticed trouble and jumped in the pool to save the child. Audio-video contractors working in the home also responded to the scene, performed CPR then rushed to the hospital.
Doctors performed a series of tests and said the boy will be OK.