The Chelsea starlet, 19, currently on loan at Derby, is being tipped for Three Lions stardom.
He revealed to The Sun: “It was tough to turn down Pompey. I was a massive fan, season-ticket holder and grew up watching them.
“Southampton wasn't as tough! I went there and I had to wear the kit and just didn't want to in training. No disrespect but I am a Pompey boy! I found it too hard wearing that kit.
“It was hard turning Pompey down but the difference between them and Chelsea was massive.
“Chelsea were a year or two ahead of what Pompey were doing with the facilities and the boys, who were so competitive."