Dylan Mackie (22) battered the boy, aged just four-and-half, repeatedly round the head and body after being left in charge of the youngster while his mother went to work.
Falkirk Sheriff Court heard yesterday the child was completely uninjured when his mum left on the morning of April 16 last year, but a matter of hours later had “extensive” bruises.
Neighbours described hearing banging and a voice saying, “stop it, stop it”, coming from the flat in Grangemouth, where Mackie was supposed to be caring for the boy.
He later took the child with him when he went to work himself, as a pressure-washer.
Giving evidence by video link from a side room at the court, the boy, now five-and-a-half, was asked what Mackie did at work.
He replied: “Hitting me.”
In an earlier joint police/social work interview, he said he had also been assaulted by Mackie in the flat.
He said: “Dylan hit me. He hurt my face. Dylan did it in my bedroom with his hands.”
The boy’s mother described her son as looking “like a battered wean” when she undressed him for his bath that evening.
Jurors were shown what a sheriff described as “graphic” photographs of the boy taken afterwards, including “a significant facial injury”, bruising to his left ear, bruising behind both ears, and extensive bruising to his buttocks.
Police found a partially-burned plank at the rear of the boy’s home in Grangemouth, Stirlingshire.
Consultant paediatrician Michael Colvin said the shape of the charred end of the plank fitted the shape of the injury on the side of the boy’s head.
However, no forensic evidence could be find to link it to the crime.
Dr Colvin told the court the boy’s injuries were “some of the most extensive non-accidental injuries” he had ever seen on a child.
Mackie denied assaulting the boy “with his hands and other means unknown”, to his severe injury.
He also denied assaulting the boy’s mother in an incident three days later, pulling her pony-tail, causing her to flee their flat “screaming for help, terrified, distressed, and barely able to speak”.
He told his solicitor, Simon Hutchison, that he “accepted” he had struck the boy on the buttocks after he soiled himself, but insisted the injury on his head must have been caused by a fall.
After a three day trial, a jury of nine women and six men took less than an hour to find Mackie, of Kersiebank Avenue, Grangemouth, guilty of both charges. The verdicts were by majority.
Mr Hutchison said the verdicts were “clearly a serious matter” but that Mackie had been on bail for 12 months since the events, with no further incidents.
He asked for bail to be continued while social workers prepared a background report.
But Sheriff Craig Caldwell told Mackie he had committed “an extremely serious” offence.
Revoking bail and remanding him in custody ahead of an “inevitable” jail sentence, he told him: “The evidence has been, and was clearly accepted by the jury, that you engaged an entirely unprovoked, violent, and persistent assault on a very young child.
“There’s no childhood transgression that could ever explain or justify it. It was down to no restraint on your part that this poor child didn’t suffer even more serious injury.”
There were shouts on the public benches as the sheriff finished speaking, and police ushered a man from the courtroom.
Mackie’s mother, who was present throughout the trial, put her head in her hands and wept.
Mackie called out: “Mum, I love you, see you soon” as he was handcuffed to a security guard and led down to the cells.
He will be brought back to be sentenced on May 10.