EPI Pets and Fat

[Montgomery]

The naturopath Montgomery saw in December would like him to be eating something else, because he feels the food he's currently on is irritating his insides. So far, we've lost the kibble, with very good results, but the wet food is proving more difficult.
Is it normal for these little hairballs to have difficulty digesting fat? One of the foods recommended for him is Tiki Cat, but we never got past the smallest teaspoonful mixed with his usual food. The diarrhea just kept getting worse. The third ingredient is sunflower seed oil. Very soon after stopping this food, his stool returned to Montgomery normal. His Science Diet does have fish oil in it, but its way down the list. The Purina does not list any oil. Both are very low fat foods.
There aren't a lot of options for him. He is not supposed to eat plants, especially tapioca. He cannot eat turkey. I'm not sure what to try with him now, especially with the shortages.
He is on tylan for SID.
Since we ditched the kibble, he has much less gas, and has far less difficulty going to the bathroom. I'd like to make a wet food switch to see if we can get him even better. Ideally, by the time he turns eleven next year, I'd like him to be housebroken!