Confused over difficulty in changing food

Over approximately the past 4 months I have been gradually changing my 10 year old cat from Cat Chow Indoor to 4Health All Stages. After research on here that seemed like a good food for the price. I am not yet up to 100% 4Health food yet. I recently increased it and it seems like when I do his stools are more loose (not diarrhea) and he will have 2 a day.

I just thought having a grain free diet would help something like that. I talked to a very knowledgeable girl that works at PetSense and she thought the grain would be upsetting his stomach but that doesn't seem to be the case. My other 2 cats seems to be doing well on it.

I guess my question is should I just change him again to something else?  I have tried 'Taste of the Wild' and I think it was doing the same thing and I didn't like the questionable things that have happened with that company. I need something reasonably priced but if I just feed it to him I could spend more. I would just like something that would agree with him. He is overweight and has lost some weight but I don't thing he has any health issues that are causing this.

Some cats just have more sensitive digestive tracts that others.  I don't think it's that it's grain free....it may be that it's simply too low in fiber. 

Is that CANNED feed we are talking about?  If so, perhaps try adding in a little bit of canned pumpkin, perhaps 1 teaspoon per meal, or up to 1 tablespoon per day.  That might do the trick.