I have no experience with heartworm but I know how heart breaking it is to see your kitty suffer.
My Persian has achne and has been on prednizone. I wonder if you mind telling me what dose he is on? It is a serious drug because it stops their immune system (as I understand it). I understand that people who have taken it for a long time get what is called moon face. and can put on weight. Also it can give mood swings.
Hope he gets better soon.