I think my cat is allergic to our flea collar!?

Ok so a few days ago i put a flea collar on my outside cat he already sneezes a lot but when i put it on him i made sure i put 2 fingers turned sideways and its a break away collar so it couldn't be too tight right? so it had to be an allergic reaction anyways there is a square on his neck about 2 inches across and 1 inch down and i looked i didn't see a big cut but i did see a little itty bitty cut anyways there is really gross green and a little bit of red snot looking stuff! well not a little bit like the whole square is green :/ i don't know what to do! i am probably going to take him to the vet but i don't want him to be put down if its serious oh and his name is pickles. please help!