The star of Disney's "Lab Rats" ... Billy Unger got fleeced by his own dad/manager, who used the cash to impress a chick ... according to a lawsuit filed by Billy.
In the docs ... Billy says his popager William Unger mishandled nearly $400K of his earnings last year. He also says his dad was gouging him for a 33.3% commission ... when the industry standard is between 10% and 20%.
Billy says his dad even siphoned away $120K to whisk away his GF on lavish vacations, and bankroll her living expenses. And it gets worse -- in the suit, Billy says his dad "despicably" took out a $1 million life insurance policy on him and made himself the sole beneficiary.
Billy's asking a judge to order his dad to repay the missing money, plus damages.
For his part ... William Unger claims his son's accusations are "false and frivolous" -- and he blames the lawsuit on a "manipulative negative force in [Billy's] life whose sole focus is to administer pain." He added, "I love Billy with all my heart" ... and he wants to rebuild their relationship.