Hi again, Vicky, and thanks for your additional information.
Regarding vaccinations, yes, cats (especially those that spend time outside with stray, unvaccinated cats) need to be tested first, for FIV (feline AIDS) and FeLV (feline leukemia virus), and if those test results are negative the cat will be vaccinated against those diseases, plus, feline rhinotracheitis (cat flu), and distemper. If your cat was vaccinated by your vet, it's most likely that he received a 3 or 4 in one vaccine called FVCRP, which covers all of the necessary protection against these deadly feline illnesses.
I assume your cat is neutered. If not, and he's not fully vaccinated or up to date on all his vaccinations with yearly boosters, he's mating with the female strays who may carry communicable diseases.
If the discharges from his nose and eyes are clear, that's good! If you notice that they turn greenish-yellow, that indicates infection. Your cat most likely has an upper respiratory infection and yes, it can resolve on it's own within a week, like the last one did, and even though a 'kitty cold' is viral in nature, most vets do prescribe antibiotics to prevent secondary bacterial infections. I would recommend a vet visit at this time, to help your boy feel better, and he might also be given an antihistamine to help his runny nose and sneezing. It's also possible that he has allergies, but if this just happened a short while ago, and now has reoccurred, he might need to be treated by the vet with medication to prevent his allergic reactions.
Cats won't eat what they can't smell, so I'm not surprised he has trouble eating due to his stuffy nose. You can help decongest at home by running a humidifier and also run a hot shower to steam up your bathroom and sit in there for about 15 minutes at a time, every few hours. Make sure he doesn't get a chill when you come out, and wrap him in a towel, if he'll allow. Also, very slightly warm some 'smelly' variety of canned cat food (seafood flavor) in the microwave, and the aroma might entice him to eat more.
Please check on his vaccination status, as his contact with the strays may be making him sick at this time. Also, you can call your local Humane Society, ASPCA or Animal Control and ask to borrow a 'humane' trap, which is baited with food, to trap the strays, have them neutered, checked over medically, vaccinated, and returned to the neighborhood. Programs like this will help you find low-cost vets in your area to spay/neuter these cats.
I hope he's feeling better very soon!