You might want to transition him over a couple weeks or however long it takes. If not, he might get GI distress. What helps with transitioning is a tablespoon or so of pure canned pumpkin or ground psyllium and giving a multistrain probiotic with meals. There’s even a product by The Honest Kitchen called “Perfect Form”. You can also google “firm up dog stool” for more suggestions. You can also transition him to a food similar in ingredients and guaranteed analysis to the Purina which might make it easier.