Pete Wentz and Ashlee -- Custody Battle Underway

3/8/2011 4:33 PM PST

Pete Wentz and Ashlee -- Custody Battle Underway

Pete Wentz does NOT want his soon-to-be-ex wife Ashlee Simpson to get primary physical custody of their 2-year-old son Bronx -- as she's requested -- and now he's taking her on in court ... TMZ has learned.

Wentz just filed legal papers in response to Ashlee's divorce petition ... asking the judge for JOINT physical and legal custody of their son.   In her divorce petition, Ashlee checked the box that indicated she wanted primary physical custody.

According to the docs, filed in L.A. County Superior Court and obtained by TMZ, Wentz also wants each party to pay their own legal fees ... even though Ashlee asked the court to make Pete foot the bill for both sides.

And while Ashlee didn't specify the official break-up date in her papers -- Pete says the two called it quits on Feb. 9 ... just days after Pete came home from a wife-less trip to Dallas for the Super Bowl.