Tameka Foster's lawyer has filed new documents in the couple's bitter child custody war ... slamming Usher ... who asked the court to delay the next hearing in light of the death of Tameka's son, Kile, who was killed in a jet ski accident.

In his request, Usher claims he's worried the hearing would cause "unnecessary stress and strain" for both himself, Tameka and their two minor children "in the immediate wake of this tragedy."

Usher also claims Tameka would get an unfair advantage in the hearing because the court would not want to rule against a woman who is still grieving the death of her son. Usher fears he could lose custody of the children and believes it would be unfair to him.

But Tameka claims Usher is full of crap ... and in docs of her own, insists U's claims of suffering "stress and strain" are "BOGUS."

In fact, Tameka claims Usher didn't care about her son at all in his final days -- even though Usher regarded him as a son back when they were married.

"[Usher] visited Kile in the hospital only once" during his 15 days on life support ... Tameka claims.

She also claims she had begged Usher to tweet a message about her son, "Please pray for Kile" ... but he refused. Instead, Tameka claims, Usher's next tweet was a photo of his own breakfast with the caption, "Breakfast of Champions."

In her docs, Tameka claims Usher's attempt to postpone the hearing is "simply a transparent sham" ... and asks the judge to DENY Usher's request and require him to appear at the next scheduled hearing, set for August 13.