What to do for soft stool or digestive issues?

My cat is about one year old, and he's always had soft stool problems. He has access to dry food (Wellness Core) all the time, and is fed canned food (different types of grain-free) twice a day, about 1/4 of a 5 oz. can each time. Most of the time, his poop is fine. But occasionally, it would be soft (not completely runny). 

I would rather have him eat wet food only, but I tried that once, and he couldn't take it. That's why he's only getting a little bit of wet food twice a day. 

Should I try non-grain-free food? Or food designed for sensitive stomachs? Is his food too high in protein? Maybe pumpkin? 

Thanks!

I just briefly posted about my Ruby's digestive issues in another thread, http://www.thecatsite.com/t/281149/for-gods-sake-when-will-the-madness-end/30 so I won't repeat it here, but Ruby who is about 3 was adopted from the SPCA in November.   She always had loose stools and would throw up a clear liquid a few times a week.   She was eating IAMS canned and dry food.   The vet recommended Royal Canin hypo-allergenic wet food only and it cured her loose stools overnight (literally) and she no longer throws up the clear liquid.   So it is definitely worth trying a food for sensitive tummies.

I think we first need to see if your cat is allergic to certain proteins, like chicken or beef. Check all of your cat food ingredients to see exactly what is in the food and try one with a Limited Ingredient Diet, like from Nature's Variety Instinct or Natural Balance. Stick to the one protein only for at least a week or two to see if it helps before slowly introducing another protein. Chicken, beef and fish are usually highly suspect because they are found so commonly in their foods. Another thing you could try is to add some probiotics to his wet food to see if it helps with any digestive issues. You can use human grade probiotics in capsules by dumping out the contents (powder) into each meal and add a bit of water for extra moisture he's lost from the loose stools. Most cats will not mind the taste. They also sell pet-grade probiotics in pet stores as well, but can be more expensive.

I hope your cat gets to be his "regular" self soon! Healing vibes sent!