If he thinks it was stolen, go to the police station to report it. They will then give him/her a crime ref no. then report it as stolen to the phone company and the insurance co.
i think he thinks hes just lost it to be honest...but he really doesnt know...could have been pickpocketed i suppose...it was just gone when he got home...
get a hold of his provider, that way no-one can run up astronomical bills.
if a PAYG then the thief can top it up.
Even if left on the bus, cancel everything you can think of and phone the bus co. in the morning to find out if anyone has had the decency to hand it in.