Well, he definately needs a dewormer. He'll need a full run of tests from the vet. Why is he three months old and not seen a vet yet? The vet would have found worms on a previous visit as basic tests, and given him the meds long before they got large enough to be visible on you.

The only thing you can do is make a regular vet appointment, get all his shots, and a full spectrum worm and parasite test. Then while you are there, make an appointment to have him neutered. He'll be 6 months old soon enough, and if you can't get an appointment for deworming him now, you most certainly won't get one in a snap to have him neutered.

And no, the Humane Society doesn't have enough funds to handle the full expenses on all of their own strays, they don't need to be responsible for yours too. Get the kitty to a vet! It should have been done months ago, and he shold have gone back a few times for additional shots.