The ex-judge’s son who killed his mother and himself at a ritzy Upper East Side co-op was a rage-fueled layabout who worried his family by dropping out of college to spend his time drinking and smoking pot, police sources said Wednesday. 

The parents of Doug Solomon, 26, had even discussed what to do with their troubled boy the night before he launched a horrifying attack on his mom, Diane Gallagher, 65, in their posh high-rise — in which he bashed her to death, apparently with a lamp, and then jumped naked to his death, law enforcement sources said.

He was found bloodied in the courtyard of the East 79th Street building with a broken back and a series of bruises that indicate he squeezed through a narrow window in his parents’ 16th-floor home, the sources said.

His father, former Manhattan Supreme Court Justice Charles Solomon, told investigators that his wife and son were asleep when he left for work on the day of the tragedy and said his family had no history of domestic issues. 

But trouble was brewing inside the family’s stately home.

The night before the grisly murder-suicide, Solomon and Gallagher talked about their son’s life, which sources say was rudderless.

The only son of the esteemed former justice – who presided over a litany of high-profile cases, including P. Diddy’s 2001 nightclub shooting trial – had been struggling since he flunked out of college about two years ago.

He’d quit sports, began drinking heavily, used marijuana and was known to have a bad temper, law enforcement sources said. It wasn’t clear what sport he quit playing, but his Facebook page indicated he played high school lacrosse at Poly Prep Country Day School.

While he didn’t have any known psychiatric history and wasn’t prescribed medication, he saw a therapist shortly after leaving college but didn’t continue seeing her, law enforcement sources said. 

Doug Solomon is said to have had a closer relationship with his mother than his father, and he rarely fought with her, sources said.

The Office of the Chief Medical Examiner on Wednesday ruled Gallagher died of blunt force injuries to her head, neck and torso.

Doug Solomon’s cause of death has not yet been revealed, but injuries to his body indicate he squeezed through a narrow bedroom window before jumping down to the courtyard, sources said. 

When cops discovered Gallagher’s body, they found a bedside lamp close to her. Police initially said a piece of furniture was used, but now suspect the lamp was the murder weapon. 

A former classmate who went to high school with Doug Solomon and played lacrosse with him told The Post his buddy was never known to have violent outbursts and called him a “very kind, genuine and light hearted individual.”

“I think it’s quite obvious he may have been suffering from depression or some other mental illness and was using alcohol and weed as a way to cope with his problems,” the friend said.

“[Him and his mom] always seemed to have a good relationship like others have said and I just wish things didn’t turn out the way they did because this is just a terrible thing for anyone to go through.”

Back outside the tony high-rise Wednesday morning, a half-dozen detectives were seen milling in and out of the building, clutching brown evidence bags. 

Neighbors who knew the family said they were shocked to hear what had happened. 

“I would have been in a position to hear commotion or fighting coming from their apartment, but never, never, in all the time I’ve lived here,” a neighbor who’s lived on the same floor as the Solomon family for a decade told The Post. 

“Never any kind of disturbance — just quiet.” 

Additional reporting by Tina Moore and Jack Morphet