Advantage Reaction

I have a community cat named Flint and I had given all the cats their dosage of Advantage yesterday, this morning at feeding time, I noticed he has a nasty red sore right between his shoulder blades, I know I directly put the Advantage on the back of his head but it may have possibly run down his neck. Anyways, I brought him in, washed him twice and rinsed him with a great deal of water. I was actually not scratched badly. I did put some hydrocortisone cream on it and am watching him closely. He is eating and drinking, even used a litter pan. He is in a large plastic dog crate that's covered. Anyone have any adverse reactions with Advantage recently? I've searched this site and seen several but they were years old. I did the call the Vet, they said as long as he is eating and drinking, to keep an eye on him. They did suggest the hydrocortisone cream.

Wanted to update he is doing better, eating and drinking, along with not scratching as much. It is about the size of a half dollar.

I was wondering if anyone knows of any flea repellants that are not pesticide based that are safe for cats? Ideally looking for something water proof as he is a community cat but if not, we can work around it.