Following up his double against Stoke earlier this month, Son put Spurs ahead after seven minutes and added his second midway through the first half before Boro reduced the deficit through Ben Gibson after the break as the visitors ran out 2-1 winners.
Pochettino said: "It is difficult for all the players who come from outside England because the Premier League is the toughest league in the world and they need time to settle.
"After one year and the summer he is a different person - he's more mature and he knows the league and he's settled in fantastically now. When a front player scores goals it is easy for him to settle."
With the extent of Harry Kane's ankle injury set to be assessed this week, Pochettino admitted some concern over his side's failure to convert their sizeable percentage of possession into a more convincing win.
"I was a little disappointed because we created a lot of chances and we weren't clinical enough in front of goal," added Pochettino.
"We have to work on our percentage and we have to work on that for the future. Two-nil is not a score you can say 'okay it's over', and at 2-1 we suffered a bit but I think we deserved the victory."