Kyle Glover, the 11-year-old son of the singer's ex-wife Tameka Foster, was sitting on an inflatable raft which collided with the watercraft on Lake Lanier in Atlanta, Georgia.
He suffered head injuries and was taken to hospital in a critical condition but reports emerged suggesting Glover had shown no brain activity since the accident.
Usher and Foster have yet to comment, but the R&B star's showbiz friends have taken to Twitter.com to offer their support to the former couple.
In a post on her page, Braxton writes:
My heart goes out 2 @TamekaRaymond thru this indescribable time.Im praying 4 u,Kyle,Ryan&ur family may God bestow his Blessings upon u all.— ASHANTI (@ashanti) July 9, 2012
My prayers go out to Usher @UsherRaymondIV and his family. His stepson was declared brain dead after a jet ski accident. Please pray 4 them.— Donny Osmond (@donnyosmond) July 9, 2012
Tia Mowry adds:
My prayers go out to Usher and his family!— Tia Mowry (@TiaMowry) July 9, 2012
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