He needs to see a vet and he NEEDS vaccinations.. Why do you not want to get vaccinations that are required for his health and survival? Rabies is required by law, but they also need the FVRCP, FeLeuk vaccines too. They are not expensive compared to what it would cost to treat the cat if he/she actually got sick.

Internal parasites needs to be diagnosed by a vet so you know what type of parasites the cat has. If you just treat for one thing, such as roundworms, but the cat has tapeworms(from the fleas most likely), then you're not going to do anything to get rid of the tapeworms. Deworming is a minimal cost that can protect not only the cat but your family. If that cat has hookworms, YOU can get them. Did you know they burrow through the soles of your feet if you step in cat feces and don't know it? Yep!

He/she will need a series of vaccinations, usually 2-3 weeks apart when they are kittens. He/she will also need to be neutered at 3-5 months of age, but that is also a very minimal expense.

Remember to also feed a very high quality food so the cat doesn't come down with urinary tract infections or urinary stones that block the bladder and make him unable to urinate.

Definitely don't allow him/her outside until vaccinations are finished, and even still, they really should be kept inside constantly, for their safety.

Good luck with him/her!