I agree with jusgal. Legally, you have no standing unless you
have documentary evidence that the guitar and gadget belong to you,
and alo documentary evidence that he borrowed it from you. If you
have warranty cards for the items with your name and the items'
serial numbers on them, you might be able to get the police
involved in recovering them.
Failing that, approaching the parents is your next best option. I
can understand why you would say that it seems childish for a
17-year-old to do so. However, at the same time, this fellow
doesn't seem to want to deal with the situation, and you will need
to do whatever it takes to force him to front up. Given that, and
given that you wish to keep your methods above board, talking to
his parents is a reaonable course of action. If anything, he is the
one who is childish if, at this age, he needs his parents to tell
him the right thing to do.
