loose poo is pretty common i think when there's a change of food. to help firm it up and prevent him from losing too much good gut bacteria, you can add some pure canned pureed pumpkin (@ 1 tablespoon per day at least - altho our Angel has gotten used to having a tablespoon with her breakfast AND another tablespoon with her dinner - any less and she seems to have a bit of trouble now) as well as some probiotics. the pumpkin also really helps to pass the hair out of her system so she doesn't end up coughing up hairballs. just a little bit of the probiotic powder works good mixed in with the food.
but, of course, also just make sure you're changing her food really slowly - we changed over to grainfree wet from friskies wet over the course of a month probably. a sudden change would shock even OUR human digestive system.