"We watched him an awful lot for QPR and were impressed," Pompey academy chief Paul Hart told the club's official website. "In the present climate there are few decent young English goalkeepers, but for a young boy Liam is mature and confident.
"We've beaten off Arsenal and Aston Villa to get him, which shows where we're aiming to be. He was particularly attracted to the idea of coming to Pompey with Harry Redknapp, David James and goalkeeping coach David Coles.
"It appealed to him that he would have a great mentor in David James."