Please don't stick anything up his rectum, it can really do more hard then good. Here's a thread with a very similar situation, I hope it can help you.
http://www.catforum.com/viewtopic.php?t=4770&start=0
To summarize the thread, it was recommended that you add about 1/4 tsp of psyllium powder (health food store) or rice bran to wet food to add fiber. Be careful with how much you add, however, because too much will just cause the cat to become constipated again. Also, you can give a finger-full of petroleum jelly as a laxative. Also, I would suggest that you call your vet again if this issue doesn't resolve, because being constipated *cannot* be comfortable for the cat!
http://www.catforum.com/viewtopic.php?t=4770&start=0
To summarize the thread, it was recommended that you add about 1/4 tsp of psyllium powder (health food store) or rice bran to wet food to add fiber. Be careful with how much you add, however, because too much will just cause the cat to become constipated again. Also, you can give a finger-full of petroleum jelly as a laxative. Also, I would suggest that you call your vet again if this issue doesn't resolve, because being constipated *cannot* be comfortable for the cat!