We somehow ended up taking on ducks many years ago at our rabbit sanctuary and still have them.

We do feed them a mix of duck pellets and cracked corn. As veegan points out they do eat a lot of bugs and can have produce (and I agree about no iceberg lettuce). I don't think the dog food is a good idea at all. I would also say that the runny poo is a big warning sign that the duck is having trouble with the food you are giving him, so you need to change his diet quick or you may lose him. If you can find an exotic pet specialist vet, they may have some ideas too. You might also try doing a search on the internet re ducks and their care. Good luck!