Downing became the club's top scorer this campaign with his third strike in the Tees-Wear derby against Sunderland on Saturday.
"I spent a lot of time last year trying to convince Stewart that he is capable of scoring a lot more goals," said Southgate.
"He scored twice last year and could have done better but he needed the belief and the confidence that the goals would come.
"Now he realises that if he keeps getting into position and having a pop at goal that they can continue to come.
"He's got three already this season and you can see the difference in him when he gets in the goal area.
"As a wide player you are going to get two or three opportunities in every game.
"I can't see any reason at all why he can't get to double figures this season if he continues to keep the bit between his teeth."