You sure he isn't just fluffed up more than most? You can see there's snow on the ground. To keep warm the birds stick their feathers out which makes them look really fat but it's only the feathers. If you compared birds in summer to birds in winter you'd think they all got really fat really quickly when the snows came but they'll actually be skinnier than ever when warm weather hits and they leave their feathers flat.
Thats what I orginally asumed when I first saw him. There are lots of birds that come to that feeder and they are a puffed too but nothing compared to him. If he is just puffed then thats good. I hope he doesn't have anything actually wrong with him. I thought it was a funny pic though.