Depending on how frequent it is, it may just be natural thing for him. If its happening a lot it might be as other poster's mentioned - a food issue.
My Winston has pretty severe issues with grains. When I first got him (and hadn't switched foods yet) it was so bad that I had to have all the windows in my apartment open 24/7. With the windows closed, it was as if something was sitting and rotting in my apartment.
In our case, switching to a food without grains was the trick and the gas issues stopped completely. You're feeding the limited ingredient potato and duck right? Other then wondering if he has an issue with potatoes or duck - flaxseed can be an issue for some humans (from a gas and bloating standpoint, sometimes with diarrhea.) If he had been fed a food with a smaller amount of flaxseed and has jumped to a much higher amount this could be causing his gas issues.