The 16-year-old turned down an apprenticeship with the Pilgrims and has agreed a youth contract with the Championship leaders.
Saints Under-18s boss Jason Dodd told the club website: "We're very happy to have him here. He's something a bit different to what we've got.
"The pleasing thing is that he's already familiar with us and likes the way we operate.
"His parents came down and watched him play his first game for us at the weekend, and I think the reputation we have of pushing players into the first team when they are ready, and on to international recognition, was a big attraction.
"He had a few Premier League clubs after him but he's made the right choice in my opinion."
