Very Scruffy Robin

Hi

I have a question regarding a robin that has made our garden his territory. He is by far and a way the scruffiest looking bird I have ever seen. He vaguely resembles a flying tennis ball and appears to be missing a large number of feathers from his head and neck. Apart from that, he seems healthy, surviving the cold, feeding well, seeing off other, better looking, robins etc.

Does anyone here know what could be wrong with his plumage? I've included a photo (I have more which show him preening and reveal a very bare neck).