Bengals and Diarrhea

I have a new Bengal he was a give up by a owner who did not know what she had
gotten into with this breed, he was also urinating on her bed. He doesn't do
that at my house and I 've had him since Christmas. He has had a problem with
diarrhea and has had it since I got him. He has been checked by the Vet and
tested for parasites and nothing found. Now he is on food trails to see which
food he can tolerate. Has anyone else had this problem and if so what was
done about this issue.