Got one of these for the 9 y/o last Christmas and he loves it:
Garmin Vivofit
They come in a variety of Disney/Marvel/Starwars characters. Step counter is great and you can ‘gamify’ chores from an app. I.e we set him some chores (tidy his room, music practice, homework etc) and he completes them he gets to access a bit more of a marvel game on his tablet.
Good basic fitness/steps tracker too. Also, and this is the winner for me for a 9 y/o it takes a battery (rather than needing charging every few days) and apparently this lasts over a year-no sign of needing replacement yet!
Edit: doesn’t need a phone for day to day operation btw, but if he has access to a tablet etc you can put the app on there.