Hi, you might want to check the shelters in your area and see if there is a low cost neuter/spay program available. Some communities have this available for low income families that want to get their pets spayed or neutered. You might also check with the local vet clinics and see if any of them offer a program. Thankfully, neutering is ALOT cheaper than spaying so that is one thing in your favour. You might be able to find a clinic that would allow you to pay half of the bill up front and pay the remaining half in post dated cheques.
The only other thing I can think of is to try and sock away 30.00 a month if it's possible and in a few months time, you should have enough to get it done. A couple of years ago, a cat neuter where I live cost about 130.00 for everything. The thing is, you usually have to have an exam and vaccines as well. But if your cat is an indoor cat you could request that he not be vaccinated. If he does go outdoors, hopefully you can keep him inside until he is fixed so he doesn't contribute to any unwanted litters of kittens that could end up living a hard life.
p.s. one other thing you could try is to have a mini garage sale and sell stuff to raise money for your cat. You could make signs so people know that the funds are going to help your cat and they may offer more than what you are asking for certain items.
I hope everything goes well and that you will find a way to get him neutered. It will keep him from straying as much and will lower his risk of prostate cancer and of course, keep the unwanted cat population down.