Usher's Stepson in Critical Condition After Jet Ski Accident

Kyle Glover -- 11-year-old son of Usher's estranged wife Tameka Foster -- is in critical condition in an Atlanta hospital after he was struck by someone riding a jet ski ... TMZ has learned.

According to our sources, Glover was sitting in an inner tube on Lake Lanier when he was hit in the head. We're told Glover was unresponsive when he was taken out of the water.

Our source say Glover remains unresponsive and is currently in critical condition.

We're told that despite Usher's acrimonious divorce from Foster, he chartered a plane for Tameka, who was out of town, to be by her son's side.

Our sources say she was extremely thankful for Usher's grand gesture and she rushed immediately to be by her son's side.