The child's body parts were discovered Monday in four plastic bags at an abandoned housing development, stunning Thailand and becoming a national media sensation.
The suspect, a 40-year-old Bangkok resident, fed the media frenzy by turning himself in Tuesday evening at the headquarters of a Thai television station where he was arrested on charges including premeditated murder. Police then allowed him to hold a televised news conference.
The suspect, Siriphong Kanchannaniwit, said he committed the two murders over the weekend while he was driving the woman and her two children in a borrowed taxi, said Bangkok Deputy Police Commissioner Maj. Gen. Amnuay Nimmano.
Siriphong told police his girlfriend, the child's 38-year-old mother, had been pressuring him to kill one of her ex-husbands, said Amnuay. Siriphong said he refused and the woman insulted him, sending him into a rage.
"She kept insulting me, saying I had no guts," Siriphong said calmly during the Tuesday news conference.
Siriphong told police he pulled out a gun and started firing shots, which killed the boy and the mother, identified as Sunan Srisuwan.
"He said he did not intend to kill the boy who was shot when he was shooting the mother," Amnuay said.
He told police he dumped the woman's body on a side street where he spotted a police car and panicked. He said he then drove home, dismembered the boy's body and divided the parts into garbage bags, which he disposed of the following day, Amnuay said.
The mother's body was discovered Sunday and a garbage collector found the child Monday, first opening a bag that contained his head.
"I immediately informed the police, and they found the limbs and the torso in other bags," the garbage collector, Pratin Panhouy, told Thai television Channel 3. An initial investigation showed the boy was shot at close range in his limbs, his stomach and his face. The boy's fingers were all missing.
The boy's 13-year-old sister who was in the car was wounded during the shooting. She was taken to hospital after the man turned himself in and police say she has corroborated the main facts of his story but several questions remain.
"There are many holes and inconsistencies so we are not going to base the case solely on what he said," said Bangkok police chief Lt. Gen. Worapong Chiewpreecha.