The boy’s grandmother witnessed the attack, about 6:45pm at their home in Moo 5 village, tambon Krab Yai, Ban Pong district.
She told police that her son and his wife had separated when the boy was young, leaving him in her care.
The boy was in Mathayom 3 at a school in Ban Pong district, and had mental problems, she said.
Yesterday evening the boy was playing with his smartphone. His father had berated him, telling him he should be doing his school homework. The boy had grabbed up a foot-long knife, which was lying by his side, and stabbed his father in the left side of his body, the grandmother said.
His father staggered away from him and walked into the kitchen, where he collapsed and died.
Police were considering legal action against the boy.