Sounds like tape worm to me. The only worms that heartworm preventatives like Interceptor and Sentinal don't take care of are tapeworms. They look like little pieces of white stuff and those are the segments that have fallen off the worm. Also, gross as it may be, pull up his tail and see if there are little pieces of ricelike stuff near his butt. As long as the worms are taken care of they are not a danger. Pretty routine stuff. Good luck and let us know ok? Way to go Binxy boy...got Mommy scared again!