My cat is really poorly, please help!

So glad I found this website. OK my cat Joey-George is just over 1 years old, I've had him and his sister since December when they were going to be taken to an animal shelter. 

Joey has recently in the last 4 - 5 days had some problems which includes peeing a lot, he also eats a lot at the moment too. He 80% of the time has runny poos but not always and he just doesn't look very happy at all at the moment. I mean he is still a very cuddly cat, he still comes for his cuddles every morning in bed and purrs his head off etc but I am very worried about him. 

But this is the difficult thing. Lily-Rose (Joey's sister) has never had any problems but for the first few weeks that we had them Joey was being sick every now and again, usually at night. He also seems to have problems eating most cat food, especially Whiskas cat food. We have recently moved again about 3 weeks ago and his meat keeps changing which I know isn't good for any animal but I can't seem to find one that is OK for him. 

I've asked questions on another site and was told that if I didn't take him to the vets he would die and but I would really like a second opinion. Like I can't figure out if Joey is ill from an illness or a problem inside him or if it's because I keep changing his food? 

it could be because of the frequent food changing or just the types or brands you feed him. it could also be a more serious problem like, colitis or hyperthyroidism so i'd take him to a vet and get him checked out

Some questions for you: are they indoor only or indoor/outdoor cats? Have they been neutered/spayed? Have they been dewormed? What is their regular diet, dry and wet or just wet? Which brands?

It could be that he ate something that upset his system although that should really have started clearing up by now. He could have a food sensitivity to one or more ingredients. His system could be upset from frequent changes in food although this generally happens switching dry to dry or dry to wet. Switching wet foods doesn't generally cause a problem unless there is some kind of food sensitivity.

The fact that his sister isn't having an issue doesn't necessarily mean much even if they are related. Since cats are individual, it means that he has a problem that his sister doesn't.

With any new change in behavior that can't be explained by obvious answers, it is best to get a vet check done. You could wait a few days to see if the problem clears up on its own if it doesn't seem serious, but if it doesn't improve, then a vet is in order.