Thank you for trying to help this skinny one.
When a stray is so skinny, it is probably due to both malnutrition and worms. Unfortunately, waiting to see whether he improves or not really has nothing to do with his need to see a vet for the following reasons.
1) The problem with de-worming medication that you can purchase anywhere but at the vet is that it does not kill the worms, it merely expels them - and it is only round worm. The problem with this is that it increases the risk of other cats contracting them when they otherwise might not, and it also does not adress the problem of the larvae, which live in the cat and then become worms. So the larvae of the worms kitty had/has need to be addressed as well, and there is no medication that can be purchased at a store that does this.
2) Even the medication given by a vet has to be administered at least twice - three weeks apart - and the type of medicine is dependent upon the type of worm(s) present.
Our little Flowerbelle was infested with just about any type of worm you can think of when we rescued her. In fact, when we finally brought her home from the hospital, the vet forgot to check a stool sample a final time. Had hubby not liked the sound of her cough, she would be dead. We took her to a different vet for a second opinion, and it turns out she had lung worm. This is rare around here (I don't know about where you live), but she was so bad off that she was literally drowning, and had we not brought her to the vet, he estimates she had about 2, maybe three days left to live.
3) From the pics, it looks as if kitty has either fleas or ticks (probably both) - and if it's not that, then he has a cold and/or eye infection.
PLEASE DO NOT USE STORE-BOUGHT MEDICATIONS TO TREAT FLEAS OR TICKS. FLEA COLLARS CAN KILL CATS (and do and have). A product called "Frontline" or "Advantage" should be used. These can be purchased on-line or at the vet only. But they can be purchased on-line as they do not require a prescription from a vet.
However, if he gains weight, that will help him fight of diseases and infections.
To help him gain weight, feed him a very high protein diet. You can buy kitten food - this is high protein. To supplement the dry food, you can boil a chick (or some pieces of chicken) with NOTHING added. Mix in some of the broth and small bits of chicken. Cats can digest this easily, and it will help him gain weight. If you make a kind of glop out of the chicken broth and dry food, you can sprinkle a little brewer's yeast on it to boost the protein. The problem with that is he may or may not like it.
The problem I have with all of this is the eternal argument - to help or not to help - if they're not going to be spayed or neutered?
Because I have been involved with rescue for a number of years, I may have a somewhat cold-hearted position on this. But if kitty is not going to be neutered, you are helping nurse back to health a cat that will then be responsible for creating more homeless cats. He will also be open to many diseases and infections, because when females are in heat, male cats fight, and they often get seriously injured. So instead of one cat passing due to malnutrition, disease or injury, there may be five or seven - or if only some of a litter die of malnutrition, disease or injury, then the two or three that live will then have five - seven of their own of which some of each litter will likely end up dying of disease or starvation.
So while I really applaud your efforts in wanting to help this cat, please try to help him in a way that makes sense for him. At the very, very least he needs proper de-worming medication - because if you help him gain weight, you're also helping the worms.
If you cannot afford a vet visit or to have him neutered, perhaps you can find a different organization or shelter in your area that can help him? With this link, you can type in your zip-code and a list of many places in your area will pop up. This way you can call around to see if there's anyone else that may be able to assist. http://www.pets911.com/organizations/organizations.php
You can also click on the link in my signature line to see if you can find any low-cost spay/neuter services in your area.