If it's worms then the meds should work. If they don't seem to be working then there may be another underlying cause in which case you should take him to a vet (maybe a different one) and have him re-examined. Don't wait, esp if he seems to be in pain.
If you truely believe that they are "letting your cat die" or that they are ignoring the problem then you could always call the SPCA on them and report it as abuse. I mean, you'll probably get in major trouble but if it's worth it to you then that's an option. I would do it if I thought my parents were letting my cat die. It's maybe a little extreme for your situation...... I say "abuse" becasue that is what the spca considers it when a pet is need of medical care and denied medical care. You did say they took him to the vet already so it doesn't sound like they are being neglectful though. Just push them a little bit more, or at the very least YOU call the vet, tell him that you don't think the meds are working and see what he advises you to do. Most likely he will want you to bring the cat back and then you can say to your parents that you spoke to the vet and he told you the cat needs to come back. I needed to do that with my parents. I knew the dog needed to see a vet and they disregarded his symptoms so just for some back up I called the vet and they confirmed that the dog should be brought in. So try that first.
Maybe It Needs To Be Put Down..or...it Might Have An Injury...some Cats Will Get Sick From Them...you Need To Feed Him Some Meat...that Will Help.
