i'm sure some people on here can help me with this

i'm trying to track down my ex-landlord ish to get my deposit back. Funny story, not: his mum, who was our landlord for almost the whole year, died about a month before we moved out. Her son took over and was understandably a bit overwhelmed by it all and presumably doesn't really know what's going on, so we cut him a bit of slack on getting the deposit back when we moved out at the start of july.
Basically its taking the piss now, though. Every step I take to track him down has failed. I've left numerous messages on his mobile whic goes straight to answerphone every time. I've found him on facebook and emailed him there. I've gone round to speak to the new tennants who are also not responding to my requests as to his home phone number or address.

What can I do? I'm at my wit's end, so to speak.