This might sound daft, but what colour is he?
I've just been doing some research after abbymarysmokey mentioned megacolon and although a lot of what I could find was highly scientific and difficult to pick apart, I found a study that showed a direct link between the genes that produce predominantly white buns with spotting and colouring around the eyes and ears, and rabbit megacolon or 'cowpoop syndrome'.
Sadly that fits my little Dougal perfectly, and it can cause complications and blockages that prove fatal.
They say that giving adequate fluids and wet greens is usually good enough though, and most vets don't know what this condition is. To be on the safe side you can give Lactulose once a week apparently too.