The 20-year-old bagged a brace at Stadium MK to take his tally to six goals in four appearances for the Young Lions, with Michael Keane also on target.
"He is a good player, a good finisher and a pleasure to work with," Sky Sports News quotes Southgate as saying. "He has got a desire to improve all the time and he is a terrific goal threat for us.
"That said, looking at our bench tonight we have some super players I didn't put in the starting lineup that I think can cause a lot of problems.
"I am pleased with the result. I think we have got to be careful because they are young players and have done really, really well in the period of time we have been working together."
The result keeps England at the top of their qualifying group with 13 points from their first five matches.