Kim Zolciak's son KJ Biermann is challenging prosecutors to show their cards ... with his attorneys questioning the alleged force and physical injuries at the center of his sexual assault case.

The 15-year-old's attorney, Jason Sheffield, is fighting all seven felony charges against KJ ... arguing prosecutors haven't provided enough details to back up key elements of their case, according to court documents obtained by Page Six.

Specifically, Sheffield argues the aggravated child molestation charge doesn't spell out what physical injury was allegedly caused ... while the aggravated sexual battery and sodomy charges fail to adequately explain how force was allegedly used.

His legal team is also taking aim at three sexual battery counts, arguing they're virtually identical ... and claims the false imprisonment charge doesn't explain how KJ allegedly restrained his accuser.

The defense is asking for a hearing and wants the lead detective to testify ... as KJ continues to maintain his innocence.

TMZ broke the story ... KJ was arrested in August after a female classmate accused him of sexually assaulting her inside a gym changing room in April in Georgia. She alleged he forced her to perform oral sex and digitally penetrated her despite her objections.

KJ was later indicted on seven felony charges and pleaded not guilty to all of them in October.

The case has also featured other disturbing allegations ... with prosecutors previously claiming investigators found videos on KJ's phone showing him engaging in sexual acts with an underage relative. His attorney disputed that characterization ... describing the earlier incident as inappropriate behavior between middle schoolers.

KJ spent about a month behind bars before being released on bond ... and he's now confined to his mother's home as the case moves forward.