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He's gonna have to pay the piper! Dean McDermott has been served papers alleging that he owes a significant amount of money to his ex-wife for back child support.

Court documents, revealed by In Touch Weekly, show that Dean's ex-wife Mary Jo Eustace is demanding he pay five months of child support for their 16-year-old son, Jack. She signed the papers on July 31, and Tori Spelling's husband was formally served on the afternoon of Aug. 3, according to the documents.

The papers show that Dean owes $8,700 and has 30 days to pay up. If he doesn't, he could face jail time.

"He could lose his driver's license or even wind up in jail," a source told the magazine. "He's a deadbeat dad. He never thought he'd be served."

Dean has four other children with Tori Spelling.