This should be brought up to the scout master and the TC and handled from within the troop. Confronting the kid yourself is not really appropriate. Especially, if you were not present at the event and not present when he returned the iphone to your son. Not saying that it is right, but the SPL probably feels that he doesn't owe another scout's parent an explanation. I don't think I would be too happy if another parent was calling my son directly about something like this.
Important thing is that your son got his iphone back. Judgements on scout oath and law are best left to those that are in charge of the program.
I would also like to know why you or your son no longer "feel safe" in the troop? I dont think i would condemn a whole scouting troop over one incident.
No one can pass through life, any more than he can pass through a bit of country, without leaving tracks behind, and those tracks may often be helpful to those coming after him in finding their way. - Lord Baden-Powell