THE Rev. JESSE JACKSON is devastated over the headline-making mystery illness that’s struck his son, U.S. Congressman Jesse Jr. – and The ENQUIRER has learned the shocking cause of the tragedy.

“At first we thought he was sim­ply exhausted,” said a source close to the family. “But after five weeks of extensive evaluation, doctors diagnosed Jesse Jr. with bipolar disorder.”

The serious mental condition is triggered by chemical imbalances in the brain, which propels sufferers from manic highs to depressive lows.

Some insiders point the finger of blame squarely at a gastric bypass operation the 47-year-old poli­tician underwent in 2004 because it can restrict the intake of nutrients.

“It is very possible that severe weight loss from gastric bypass can cause a loss of nutrients that can trigger or exacerbate depression, bipolar disorders or schizophrenia,” explained Los Ange­les-based weight-loss specialist Dr. Carson Liu.

“These usually occur in people in their 30s and 40s after bypass surgery. We often refer patients to psychiatrists for lith­ium to help stabilize their moods. I know of one man who had such severe depression after surgery he committed suicide.”

The Illinois congressman, a married fa­ther of two, took a sudden medical leave of absence from Congress on June 10 and fell out of sight, triggering speculation about his condition.

“It’s very painful for his mother Jackie and father Jesse,” said the close source. “This is going to take a long time.”

And his dad recently showed the heartache he’s feeling. With tears in his eyes, the veteran civil rights leader told a reporter: “Please, just pray for him.”