My cats worms won't go away!!!! Pls help

I have no clue where to post this but it has to do with a cats health so I figured this would work. My cat has worms pretty badly, they come out of his butt pretty often... now he weighs a lot like 14 pounds. He's not really fat but really long and big. Just a little fat lol. But I bought some pills from the pet store and gave him 4 a day, crushed up in some soft food. Now he had them for a couple weeks and they obviously didn't work. He seems to be getting pretty uncomfortable so I think they're getting worse. Should I get a stronger dose from the pet store or is it time to see the vet? I'm pregnant and tight on money right now so I just don't want to waste money. Any advice would be greatly appreciated as I hate seeing my baby sick. Also my in laws told me to give him gun powder??? Apparently it's safe and kills the worms? Is that okay cause I'm pretty skeptical about it.