Any more pictures of him? He looks to be in great form.

My last cat had the same thing happen, the 'owner' just moved and left him and the neighbour's ( a friend's mother) dog brought him in half starved. When she died he came to live with me as her family were going to have him put to sleep. He was very clingy at first, used to sulk when I went out to work, but once he got settled and used to my routines he was fine and had a happy last few years.