hi and welcome. I will send you some sites to do tons of reading. it is very hard to transition OFF of fancy feast, its like taking candy away from a baby.... what your kitty has is flea bite dermatitis or ECG.
I would recommend stopping the once a month treatments since there is no evidence that he now has fleas and perhaps these harsh chemicals are also contributing to his problem...-(
first of all, this is a link to our 'archives' on dermatology and you can see how common this problem is, read on what has and hasn't worked for some and see what you can try. I do stress NOT to resort to steroids b/c of all the health problems they can cause and its only treating the problem and not the cause..
http://www.medhelp.org/topics/show/56
You are right in thinking that he HAS to eat, so fancy feast is better than not eating, it is also BETTER than dry food....but the best you can try to do is get him on a grain free food. b/c grains and/or chicken are often the culprit....the only way to transition over to anew food is to do it slowly...by adding 1/4 new to 3/4 old than slowly widening the gap...takes a bit of time....
I will send you a link to info on foods, it also has a link to transitioning, its on dry over to wet but you can use some of the same techniques.
http://www.catinfo.org/
good luck and please post anytime if theres anything else we can help answer...-)