Runaway pet... it's sad in a sense, but then again you want him to be happy also, right? Maybe his biggest wish is getting away and flying free, and passing by your garden once in a while to see if you're doing okay, remembering the also happy times with you.
And if you have had him since he was a little bird, would it be somewhat natural for him to kind of fly independent from his parents? Like... he kind of reached the time for going alone and start a new life as an adult bird, just like kids going away from home to go to universities, to chase their dreams, etc.
While on one hand I hope it will return eventually (my biology had a crow or something which would actually come back after flying free), on the other hand maybe you can look at the other side of the coin- he must be flying wherever he is, making new friends and having an exciting new life now!
Cheer up!
p.s. Maybe you have tried it but putting his favourite sorta seeds or food might increase the chance of his return! Trust me, runaway teenagers do miss home food sometimes.