A new sideshow is about to open in the Michael Jackson circus. The father of the pop star’s accuser plans to launch a gloves-off battle for custody of the teen tomorrow, his lawyer said.

“My client has been in hiding, basically,” said Russell Halpern. “And now he’s coming out swinging.”

Actually, his client won’t be coming out at all. He plans to skip the news conference in California because “he’s extremely camera-shy,” Halpern said.

Instead, the lawyer will do the swinging – by claiming that the 14-year-old’s mother is loonier than all the Jacksons combined.

“We’re going to be bringing up a lot of things she wouldn’t want public,” Halpern said. “She’s been locked up in a mental institution, and she suffers from psychosis.”

Pressed for details, Halpern admitted he didn’t have many. His client doesn’t know the name of the hospital where his ex-wife was supposedly treated, but Halpern said he could tell from the description it’s a “lockdown facility.”

“That’s where we keep people that are dangerous to themselves or other people,” he said.

The mother should be able to throw a few jabs in her ex-husband’s direction. He pleaded no contest to spousal abuse in 2001 and no contest to one count of child abuse the next year, court records show.

All the mudslinging could affect the Jackson sex case, because the family’s credibility is under attack by the defense.

The parents can’t even agree on whether the boy, who met Jackson while he was battling cancer two years ago, is in good health.

Comedy club owner Jamie Masada, who is close to the mother, revealed last week that the teen’s one kidney is failing.

Not true, says Halpern. “He is in the same condition as he was six months ago. He’s not close to death,” the lawyer said. “He doesn’t need to be hospitalized.”