Short answer. Alex grew up.
They all keep the memory of their adventure in the game, so what happens between 1996 and 2016?
Fridge, Bethany, Spencer and Martha were in their teens when they discovered the game. Probably born around 1996 i.e 20 years earlier.
Alex was a teenager when he discovered the game in 1996. After which 20 years has passed in real timeline of him being stuck in the game. So he was returned to his original timeline i.e 1996 when game ended. After which he grew up, now he is around 40, married and now has a kid.
Why is Alex surprised to see them and why hasn't he tried to get in touch earlier
When Alex returned to his timeline in 1996, others were not yet born or maybe toddlers. He couldn't probably know their whereabouts. He might have moved on with his life doing college, getting married, getting a job. Life happened to Alex. He might have forgotten all about it until he saw them again in 2016.
Why Alex didn't try to destroy the game?
This is unexplained as the time travel depicted in this movie is not he kind of one in The Back to the Future where you can alter future events by meddling with past.