I'd take the fact that he is on screen, the same age and still in high school as evidence that he was dusted... maybe it will be addressed in Far From Home.
Also, what other explanation is there?
Not really another good one. Have to wait and see the Spider-man movie. It looked like there were a fair number of returning high schooler's in the SM movie.
Hard to believe that the entire main cast survived. Not impossible, but it is just hard to believe. That said, the Guardians lost everybody but Rocket and Nebula.