Probably the two most common causes for diarrhea are diet and parasites, and very often when someone gets a new cat that has been living in an environment where there are a lot of other cats, that's when you suspect parasites. And of course the danger is that your other cats could pick them up from him as well.
Parasites can be difficult to detect. The standard "fecal floatation" test that a vet will do has a high rate of false negatives. Sometimes a vet will simply ignore the test and choose to treat for the most common parasites proactively.
For some reason, where it comes to bengals, like Sherry said, they often have TF, which requires a special exam that vets typically are not equipped to do (many of them have never even heard of TF). Specimens need to be sent off to a special lab etc.
Many users of this forum have had bengals with TF, and what many of them had to do was to read up and print out info on TF, and bring it to their vet.
viewtopic.php?f=9&t=3876You'd probably want to (attempt to) rule out the more common parasites first though.
If it really is a food allergy, then that requires taking a close look at the ingredient lists on the current food, and then getting some special prescription food from your vet that has none of those ingredients (typically the hyper-allergenic food has a single, oddball protein, like rabbit or venison and then very few other ingredients).. Your cat would have to stay on the prescription food for several months, and if the symptoms go away, then it can be assumed that it was a food allergy. Some people have reported that their vet was able to run tests specifically to find what the allergies are (impressive, my vet can't do that).
Good luck