Hmm, one of our guys had the same problem. about once a month he would get terribly bloody diarehha (sp?), soft and hard poops with blood in them or just blood comming out in general. He ended up having an "infection". He was given some pills for it. They weren't more than $15. But about 3 months later he ended up getting so badly constipated and in pain that we had to take him for an emergency enema (not good). The enema plus the emergency vet visit cost us over $180. We found out that it was all due to hairballs not being digested, and the infection because the hair on his bum and penis was long, so he couldn't clean it properly. So basically, we switched vets, because all she would have had to have done was give us some tonic-lax (cat laxitive which he eats daily), trim up his rear-end, and suggest a high-fiber food. but instead took a quick 2 second look at his bum and throw some pills our way (btw, the pills never cleared up the infection, the emergency vet gave us a salve to put on it daily until it cleared.)
so in conclusion, make sure you take him to a good vet and have them check him out further. It may not be parasites, but just simple hairball blockage like we had. It will give you a better piece of mind to give him some laxitive (if that's whats needed) then have it all ball up until he's hissing and screaming in pain in the living room and ending up at the emergency vet at 2 am in the morning like we did. Please make sure they feel his tummy properly for constipation, our former vet never did.
I hope this helps! And I hope he feels better!