"We noticed that he had started to elongate his vowels in words like 'bath' which he never did before. He no longer has short vowel sounds - they are all long. It's bizarre," the Daily Mail quoted Ruth, as saying.
William was admitted in hospital after suffering a fit in March last year.
"It all began with a headache. William said his head really hurt above one eye and he had a high temperature. There was a bug going around school, so my husband and I didn't think it was any more than that. But a few days later he had a massive seizure," she said.
Doctors discovered he had an abscess on his brain, known as a subdural empyema, which is caused by a rare strain of meningitis, and hence needed a life threatening operation to take away the fluid.
Following the operation William lost the ability to read and write and his memory was also affected.
After being discharged from hospital, William went on a family holiday to Northumberland with his parents and brothers Alex, 16, and Edward, 15.
"William was playing on the beach. He suddenly said, 'Look, I've made a sand castle' but really stretched the vowels out, which made him sound really posh," Ruth said.
"We all just stared back at him - we couldn't believe what we had just heard because he had a northern accent before his illness. But the strange thing was that he had no idea why we were staring at him - he just thought he was speaking normally," she added.
Brain surgeon Paul Eldridge, who works at the specialist Walton Neurological Centre, Liverpool, said it was likely that the infection and abscess had affected the area of the brain which controls language skills, forcing William to learn how to speak again.
"It's as if he's re-learnt how to talk from listening to language from sources different to those that prompted his speech first time around," Eldridge said.
Phil Edge, head of therapy at the brain injury charity, Brainwave, said: "I've heard of other patients developing changes in their speech or behaviour following a head injury or brain surgery, but not quite to this extent that an accent completely changes."
"Usually, a person's speech changes in pitch or tone, but it's interesting that this boy's lost his Yorkshire dialect completely. Obviously there has been some change to the central speech centre of his brain which has caused differences in how it is functioning now, compared with before the operation," Edge added.