This is more of a postscript since I've been neglecting the updated here for a while. Fred is offically heartworm free! He has gone through his month treatment with flying colors. No real side effects. The shots in his butt made his legs weak and trembly for a while (he still has a little tremble now and then) but I think that it was worth it. Starting next month he'll be on prevention and hopefully he'll never have to go through something like this again.
Fred really really REALLY didn't like being left at the vet. In fact after his first treatment the vet called and said that we could pick him up a day early because "he would be much happier at home." I think that he meant that they would be happier with him at home. Then we had to take him back for a one day treatment and that was it. Both times we had to go back to get Fred out of the cage cos he wasn't having anything to do with the vet techs. But I must say the people at the vet were really good at handling Fred. For as mean as he was to them they never let show that they were mad or scared at/of him. They really handled a scared and unhappy dog really well.
Now, the next phase in Fred's life will be to get fixed. We'll probably make that appointment soon....poor guy, everytime he goes to the vet he gets left...