Yes, that does look like a walking stick. I've seen this picture before and didn't notice it. That night ("P & P" concert), when the stage curtain opened, David was already sitting down. At the end of the concert, he stood up to thank everyone and then walked away normally, as he did at any of the other times I saw him again - on stage or around the venues (David is quite good at disguising physical pain, bless him). I now wonder why he was carrying the long brolly we've seen in some photos, as the weather was sunny and dry that weekend, with only a little rain on the evening of the "Cartography" concert.
If he is experiencing a declining health, it is a reflection of a man who's been through a lot of c**p (divorce and other unpleasantness) for the last 10 or so years. I have a friend who's been through a very painful divorce and seemed to age overnight. Also, living alone, maybe David isn't looking after himself as he should, eating properly, exercising regularly etc. It can be reversed though, if his life situation changes (my friend, for example, looks years younger now). With David it's his health that is worrying and rightly so. However, his musical talent and integrity shows an artist who remains totally unafraid to push and break the fence, his voice grows more and more gorgeous and...he still has very handsome, delicate features. I pray for his pain to ease and his injury to heal.