I would start out very slowly by taking out just a little bit of his old food and adding the same amount of the new, a tablespoonful maybe to start as he is so young. Gradually increase those amounts over about 10 days or so. Don't rush it! If you do it too quickly, you will probably get an upset tummy which sounds like he already did! Don't worry though, we have all been there!! I still take it very slow with my boys if I have to change, which I hope I never have to again.