dry food and runs..

i have tried to find others with this problem but, haven't. my cat is 15 years young. these past 6 months, he has gotten the diarhea from dry food. we took in a stool sample and there is no bacteria, etc. we have him on wellness moist, twice a day. this works but, he has always had dry out all day. so, he wants food more often. when we give him too much moist he gets the runs. we have tried about every dry food out there, nothing helps. just the moist a few times a day.

the can says to feed one 5.5 oz. can per 6-8lbs. of weight. well that means he should get about 1 1/2 cans a day. his limit is about a can a day (on a good day). anymore than that and he has the runs. like i said he isn't a young kitten. nothing changed in his diet to have this on-set occur. what are your thoughts on this. we do really good for a while and then we will try another dry (this is put out after eating moist) and within three days he is pooping everywhere again. if we try to give him more moist he does it then, too.

we are planning another trip to the vet this week. what kind of problems would arrise with cancer, etc.? with his age could it be something like that?