What is the fat, fibre and protein percentages of the new food? If they're quite different to what he was previously on and more higher in content, this could potentially cause watery stools if you did not switch him over slowly? Is he completely on the new food now or are you still in the process of switching him over (using some of the old food as part of a mix)?
You could try adding some probiotics to his food (I use benebac for small animals) to help settle his stomach during the food transition and see if that helps. How long has he had watery poops for? If it continues or gets worse you may need to take him to a vet. If he has diarrhea for more than 24hours I would personally take him to a vet, as you dont want him to get dehydrated from it, especially if it is currently summer where you live.