Jasper used to have very soft, diarrhea, stinky poops too when he was on a cheaper food. He did OK on orijen, but then changed to Nature's Variety Instinct and raw and now there is almost no smell to his poops. I am figuring it is the high carb content that he couldn't digest.
A seven month old kitten should be hungry enough to try different food, but the key is slow transition and put new food down when he is hungry. You would be surprised how less picky they are when they are hungry
. Think about human children.