A man has been found guilty of killing his girlfriend's six-week-old son by beating him repeatedly with a shoe and plastic bottle.

Michael John Pearce was babysitting Alfie Sullock at his home in a south Wales village while the child's mother, Donna Sullock, went for her first night out since giving birth.

Minutes after he texted Sullock reassuring her that the boy was fine and he could be trusted, Pearce dialled 999 to report that Alfie was not breathing.

When paramedics arrived at Pearce's home in Nelson, Caerphilly, they found Alfie blue and lifeless and noticed bruising to his face and chest. In hospital a scan found that Alfie had suffered a brain haemorrhage. He died four days later.

A postmortem found that the child had suffered a number of injuries to his head and abdomen. Prosecuting Pearce at Newport crown court, Michael Mather Lees QC claimed that Alfie was not hurt in a "moment of exasperation" but was "repeatedly beaten".

He said: "This was not a moment of exasperation that can happen with a screaming child. This was a baby repeatedly beaten with objects. This is not a case of a baby developing sudden death syndrome. This was a child who was badly beaten."

Pearce, a motorcycle mechanic, claimed he had no idea how Alfie came by his injuries. He said he left Alfie for "30 seconds" while he went to the toilet and noticed the child had stopped breathing on his return. Pearce also claimed that paramedics shook Alfie on their arrival, which ambulance staff dispute.

In the witness box, Sullock was asked whether she could have hurt Alfie as she got ready to go out. She replied: "No, I was always very careful. He was so small. He was a newborn baby."

Pearce was found not guilty of murder but guilty of manslaughter and faces life imprisonment.

The jury was told that Sullock, from Cardiff, discovered she had become pregnant while working as a holiday rep in Crete. The pregnancy was unexpected but she was elated. She returned to Wales and Alfie was born at the University Hospital of Wales, Cardiff, in July last year. The child was healthy and his mother's pregnancy and labour were uneventful.

Six months into the pregnancy, Sullock had become friends with Pearce, a divorced father of one. They went on a number of dates but the court was told he began to display "obsessive behaviour" towards her. He had asked Sullock if she would have a baby with him, but she had rejected the idea.

Before she went out on the night Alfie was attacked – 16 August last year – she fed, winded and changed him and left him healthy and well. At 8.43pm Pearce sent Sullock a message reading: "You can trust me you know." He told her "everything is fine" and said he was watching television. But at 9.11pm, Pearce dialled 999 and told the operator the child had stopped breathing and felt cold.

The jury was shown photographs detailing 10 facial injuries to the child, as well as bruising to his abdomen. The prosecution said marks on the defendant's shoes precisely matched injuries on the baby's face.

Sullock told the jury Pearce seemed kind and reliable and was "fantastic" with her son, as well as being a good father to his own child. "I had no concerns," she said. "Alfie was great. He was really happy."

Sullock, who clutched her son's teddy while attending the trial, later said: "We are disappointed at today's verdict but satisfied he will still go to prison for what he has done. Whatever sentence he gets will never be long enough for taking Alfie's life away."

Pearce will be sentenced at Newport crown court on Wednesday.