Bakers is an utterly terrible food, packed full of additives and very poor quality battery farmed meat. In all honesty JWB is not brilliant since it was bought out by pedigree but better than Bakers anyway. However, your issue now is not with the JWB per se but the change of diet. Whatever you do, don't go back to the Bakers. It would be like feeding your child on MacDonalds and Sunny D for the rest of its life. Do a couple of days of chicken and rice til his tummy settles down and then introduce the JWB back in gradually. If the JWB continues to cause problems or you want a really good food then look into Burns which is organic, made with good quality free range meat and formulated by a vet.