The Everton winger was part of the Under-20 squad that won the World Cup in 2017 and is expected to play for the Under-21 next summer in Italy during the Euro 2019.
However, Lookman has been the subject of concerted interest from Nigeria, whose manager Gernot Rohr is preparing for next year’s Africa Cup of Nations.
Three weeks ago, Rohr said that the Everton player “wanted to change his mind so he can play for us as he will add something to our team”.
Boothroyd spoke with Lookman, 21, at St George’s Park last week and has been assured that he is committed to representing England.
“Every time I see him I have a little joke with him about it,” said Boothroyd whose side face Denmark in a friendly on Tuesday evening. “I say: “Why would you want to leave us,” Boothroyd told dailymail.co.uk.
“But he’s very focused on England – that’s what he tells me. He says he wants to play international football for England.”