I have a very stubborn ferret?

Okay so I have 2. One of them, Momo, never poops in the litterbox. Sometimes I catch him before he goes to another corner and I put him in the litterbox and he goes, but other times he keeps pooping/peeing in the opposite corner of the litterbox. Even when I switch the location of the litterbox, he goes to where there is nothing. Also someone mentioned to put something they like at the spot so they wouldn't pee/poop there but when I did that, he just pooped on it instead. ALSO, he never liked dry food. He's already 5 or 6 months old and he just refuses to eat it so I have to soften it all the time! What do I do? :(