The 17-year-old opera singer's mother explained that he quit his A-level studies in October before his audition with singing partner Charlotte Jaconelli.
Tracy Antoine said that her son's run in the competition so far has vastly improved his confidence and happiness.
"He's a different person since that first audition," she told The Mirror. "We've met some lovely people and it has helped him enormously.
"Before this, he'd hide under a big jumper and hardly go out. Now he's putting on a Superman T-shirt and going off to shopping centres.
"It's wonderful to see him so happy" she continued. "It's made him realise there are lots of nice people out there. I know his career could take off with this, but I don't think he'll ever change. He'll still be walking along the street with a plastic carrier bag. That's just the sort of person he is."
Antoine's mother also praised his singing partner Charlotte Jaconelli for helping her son, adding: "He'd never have managed to sing for Simon Cowell if he hadn't had Charlotte by his side."
Jaconelli's mother Lisa added: "We're so thrilled. We knew Charlotte had an amazing voice, but it's great that Simon now thinks so, too."
Antoine and Jaconelli were voted through to Saturday's live final during Monday's episode.
Cowell praised the pair for their performance, saying that it was one of his favourite ever Britain's Got Talent moments. He added that Antoine's decision to keep his friend with him as a duo was the "best" one he will ever make.
Watch Jonathan & Charlotte's performance below: