The fever is of concern. Take the flea collar off and bathe your cat in warm water with a baby shampoo or mild dish detergent (such as Dawn, original formula). If you have neither of these, use a mild shampoo of your own, but dilute it with plenty of water and rinse extremely well and often. Dry in a clean, soft towel and keep him warm and out of drafts until he's completely dry.
It's important that you learn all you can about flea treatments such as collars, powders, sprays and over the counter drops before even considering them again
They don't really work and can be dangerous http://www.wildlifeprotection.net/everything/eduFleas.html and http://www.nrdc.org/health/effects/npets.asp
If your cat's fever is over 103.5, he needs medical intervention just as soon as possible. He may be having a serious reaction to the insecticide in the flea collar - so please monitor his symptoms closely (here's more about what to watch for http://www.sniksnak.com/cathealth/howto8.html )
The way we eradicate fleas at our own rescue facility? Give them a bath. Seriously, it's as uncomplicated as that. Lather the animal up, let the lather sit for 10-15 minutes and rinse well. Repeat in a couple days. We vacuum carpets daily (toss bags every other day or empty recepticles daily into airtight bags or buckets of water). Fleas can't survive in water. It only takes about 10 days to eradicate them with these measures and it doesn't cost a thing (other than vacuum bags).
good luck and God bless,