get him on tape admitting that the shit is yours and that he simply DOESNT WANT to give it back … as opposed to it being his in some way shape or form
then call the parents … if that fails … call the cops
have the cops come when the son ISNT home, and stick it to the parents saying that they have stolen merchandise in their house and they… not their son … are responsible if you want to push for legal action
if they have any sense, they will cough that shit up before the cops have time to blink