I agree raw is the best but if that is not an option for you then a grain free kibble is a must. It can take several months for his system to adjust to a premium grain free kibble after eating a brand like Hills. I don't think you gave him long enough but I also don't think a meat protein like duck was the best for him either as it is quite rich.
Do you have Orijen or Acana Provincial where you live? They are both made by the same manufacturer ( www.championpetfoods.com
) and are the best grain free brands imo ....alot of the members here feed it as well. Check out their store locator if you are interested as they also have online distributors.
Another brand is Innova EVO ( www.naturapet.com
) if you can't find Orijen/Acana Provincial. He is probably still going to have loose poop for awhile but you can add some digestive enzymes or prebiotics to help him adjust. But his itchies should go away sooner.