Alex Murdaugh’s surviving son, Buster, broke down outside court after his father was convicted of killing his mother and brother, a source told The Post on Friday.

“Buster held up pretty well until the cameras were off him — but then he collapsed,” said a source who was at the Colleton County Courthouse when Murdaugh, 54, was found guilty of the grisly double murder.

“He was crying uncontrollably. The uncles [Alex’s brothers, John Marvin Murdaugh and Randy Murdaugh] finally got him into a car.”

Buster, who’s stood by his father in the aftermath of the murders of his mother, Maggie, and his brother, Paul, on June 7, 2021, was stone-faced in court Thursday night when the verdict was read.

Toward the end of the six-week trial, the redhead took the stand in the former lawyer’s defense.

Alex Murdaugh was given two life terms for the murders of his wife and son.

Despite his overwhelming emotions Thursday night, Buster was also back at the Colleton County Courthouse for sentencing Friday morning.

He remained composed as Murdaugh was sentenced to two consecutive life terms for the killings.

Many observers of the six-week double homicide trial of Alex Murdaugh felt sorry for the 54-year-old disgraced lawyer’s sole surviving son, copper-haired Buster.

“No person with a heart and soul cannot feel sympathy for that young man,” Columbia, SC, attorney Joseph M. McCulloch Jr., a Murdaugh family friend, told The Post. “It’s just been tragedy on top of tragedy for him. I don’t know how anyone can withstand that. I’m glad he has a girlfriend who seems so supportive.”

But even more dark clouds are lurking overhead for the one-time law school student, one source with knowledge of the situation told The Post Friday.

“Buster is next … your son is next, Alex!” yelled an onlooker as Alex Murdaugh was led to a police van after his sentencing Friday morning.

The person seemingly was referring to the ongoing South Carolina Law Enforcement Division (SLED) investigation and lengthy rumors surrounding Buster’s possible involvement in the 2015 death of Stephen Smith.

Stephen’s body was found on a rural road in Hampton County, a few miles from the Murdaugh hunting lodge where Maggie and Paul Murdaugh were murdered in June 2021.

His skull was partially crushed, there was a hole in his forehead and his shoulder had been dislocated, according to documents in the case and photos his mother saw.

Based on those discrepancies, Sandy Smith believes her son’s body was deliberately placed in the road after he was murdered to make it look like a hit-and-run accident — and that there may have been a conspiracy among local officials and possibly the Murdaugh family to cover up Stephen’s death.

Both Sandy and former Highway Patrol investigator Todd Proctor, who oversaw the Smith case, previously told The Post that they believe Stephen may have been murdered — though the death was originally ruled a hit-and-run accident.

Sandy, has long been vocal about her belief that the Murdaugh family, including Buster, may have had some tie to Stephen’s death.

Speaking after Murdaugh’s conviction, Sandy told The Post: “That jury done excellent. They seen through the lies and a Murdaugh is finally brought down.

“Now that this case is back over, they can get on Stephen’s case full-time.”

Buster — aka Richard Alexander Murdaugh Jr., named after his father and great-grandfather — lives with his lawyer girlfriend, Brooklynn White, at a condo that she owns in Hilton Head, SC.

But after Buster broke down outside court Friday, minutes after his father’s conviction, he was whisked off by his uncles to what is known as “Buster’s Island” — a property on Chechessee Creek, near Beaufort, SC, that has been in the Murdaugh family since the 1970s, a source close to the family said.

“There is not a lot of warmth for Alex or Buster in Hampton these days,” one Hampton resident whose family has known the Murdaughs for decades told The Post. “I think [Buster] will stick with his uncles and stay out on Hilton Head. I don’t see him coming back here.”

Buster’s cool and calm demeanor, which he showed in the court gallery every day of the trial as well as on the witness stand, evaporated Friday.

Murdaugh, 54, was found guilty Thursday after a jury deliberated for a mere three hours. Maggie, 52, and Paul, 22, were shot dead at the kennels of the family’s hunting estate in South Carolina’s Lowcountry on the night of June 7, 2021.

“Buster held up pretty well until the cameras were off him — but then he collapsed,” said a family friend who was at the Colleton County Courthouse.

Alex Murdaugh was given two life terms for the murders of his wife and son.

“He was crying uncontrollably. The uncles [Alex’s brothers, John Marvin Murdaugh and Randy Murdaugh] finally got him into a car.”

Buster showed none of that emotion when he was grilled by defense attorney Jim Griffin, an old Murdaugh family friend. He didn’t flinch when asked about how his father broke the news of his mother and brother’s deaths to him.

“He asked me if I was sitting down, and I was like, ‘Yeah,’” Buster testified. “Then he sounded odd and then he told me that my mom and brother had been shot. I kind of just sat there for a minute. I was in shock.”

The flame-haired surviving son met his longtime girlfriend, Brooklynn White, at the University of South Carolina Law School, his father’s alma mater. Buster had been a law student there as well until he was expelled, allegedly over a plagiarism incident, the Wall Street Journal reported at the time. Local outlet, The Post and Courier reported that “the family had paid a well-connected Columbia lawyer, Butch Bowers, some $60,000 to help secure Buster’s readmittance.”

Buster eventually graduated from Wofford College in Spartanburg, SC, in 2018.

“That’s where kids from here go when they can’t get into Washington and Lee,” a local resident said.

It’s unclear what Buster currently does for a living, but he worked at his father’s former law firm prior to Alex’s arrest.

Said another family acquaintance: “I didn’t know Buster well but he always put on good manners for me. I just wasn’t sure what he was like when he wasn’t putting on those good manners.”