As someone else mentioned, URIs in kittens is fairly common. Typically, a vet would prescribe liquid Clavamox which is very effective against URIs. Most vets won't perform a neuter until the animal is well enough. If you're concerned about the anesthesia, you can consult with your vet about any pre-surgical bloodwork that s/he may recommend. Regarding the fleas, you can also ask your vet about Capstar (a one dose pill), however, if the little guy has fleas, there's a possibility that they're also in your home which would require you to do a thorough cleaning and vacuuming, throw the bag away and then repeat again in about 7 days to ensure you've gotten any eggs before they hatch. Good luck!