Son was never a "first 11" player under Pochettino. He's averaged around 24 league starts per season with us, which is heavily weighed against the fact that he was used as a backup to Kane, and got more chances with the amount of games Kane missed in recent seasons.
He was very often the player that would be playing great, but would still always seem to start on the bench behind the likes of Kane, Lamela, Eriksen and Dele.
If anything has changed him in his development, I think it's the national team. He started to lose opportunities to get around national service, and seemed to really step up his determination and aggression when (literally) going for gold. He was always an excellent ball-player, but in earlier seasons with us he would be very Eriksen-like; that is, ducking out of tackles and showing no bravery/effort in 50/50s where he would otherwise get the ball.
In my opinion, Son of 17/18 wouldn't have scored that goal yesterday, as I believe he'd have flinched out of a couple of the dribbles where he had to knock it past their defenders.
Mourinho seems to love him, and there's a very good chance he'll flourish under him, and probably end up as his talisman.