The bratty 30-year-old son of a millionaire Manhattan hedge-fund founder was charged Monday with murdering his dad after the old man cut his allowance — by a measly $200, law-enforcement sources said.

Thomas Gilbert Jr. faces multiple charges including murder and criminal possession of a weapon after he allegedly fired a single shot at his father’s head Sunday night.

The son, who just found out he was getting a $200 cut in his monthly allowance, asked his mother to go get him a sandwich so she’d be out of the family’s tony Beekman Place apartment just before he pulled the trigger and executed his dad, sources told The Post.

“He was cutting his allowance. He had been giving him $2,400 a month for rent and $600 for spending money, and he was cutting that to $400 a month for spending money,” a source said about

Thomas Gilbert Sr., founder of the Wainscott Capital hedge fund.

“They had argued about it before.”

Gilbert Jr. shot his father about 3:30 p.m. Sunday in the family’s posh pad at 20 Beekman Place with a high-powered .40-caliber Glock semi-automatic pistol that he owned, police said.

Sources said they found the box the gun came in after Gilbert Jr. surrendered at his apartment at 350 W. 18th St. about 11 p.m. Sunday. Police also found two magazine clips and loose rounds scattered around his Chelsea pad.

It is still unclear whether he bought the gun legally.

Gilbert arrived at the apartment carrying the gun and, once inside, asked his mother to leave.

“He asked her to get [him] a sandwich so she wouldn’t be there,” the source said.

Gilbert Jr. fled after the shooting and was nabbed hours later at his apartment.

Cops were alerted to the killing when the victim’s wife, Shelly, returned and discovered his body and called 911.

“My husband’s been shot by my son!” she said.

He gave himself up at his apartment about 11 p.m. Sunday after cops pinged his cellphone and tracked him down.

He at first refused to come out when they banged on the door, but eventually surrendered.

The handsome Gilbert Jr. — a graduate of the Buckley School on the Upper East Side, Deerfield Academy in western Massachusetts and Princeton — was a fixture on Manhattan’s black-tie society circuit and was often photographed with a beautiful woman on his arm.