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Don't just tell him to give it back; tell him why he should and ask him if it's the right thing to do. He'll most likely come to the right conclusion himself.
This.
He's at the age where he will understand right or wrong, not fully as kids are kids. You and his Mum still pay for him so at that age its like whatever, money grows on trees (at least it felt like that to me at his age, no worries and all that)

Maybe there has been something your son has lost and never got back, question him about that and ask him whether he would have liked to get that item back and as quoted he'll realise that giving the phone back is the right thing to do.
I remember finding a pair of reading specs/glasses in the case up the southern downs when I was younger, no mud or whatever over them so recently dropped/lost and me and my Dad went down the police station to hand them in (Dads idea). We had no need for them and I could have just slung them into a bush but we took them down, handed them in and that was that.
I never did know whether the person got their glasses back but I vividly remember it and even the small hill surrounded on both sides by bushes/nettles where I found them. I look back on that memory and realise my Dad was right for doing that and for teaching me that lesson (Police station isn't close to our house) Who knows, somebody who thought they would never see their glasses again was happy, and to know that there are people out there who do the right thing

