I'm so very sorry to hear this.
im sorry to say but he passed away an hour after i posted this. thank you all for trying to help.. you are all highly appreciated..
If you're cat drank antifreeze it would likely be dead within 48 hours (highly toxic!). Try an OTC dewormer. But remember, their are many parasites they won't work on... so follow the advice of the others and prepare for a 3 hour drive.
Take plenty of good Cd's for listening. Perhaps the soundtrack to "Les Miserables". It's a three hour play, just about puuurrrfect. :-)
If this cat is still alive it is not poisoning. It would be either a realing bad case of internal parasites or a feline virus like aids, leukemia or fip. This cat needs medical attention. If its a feline virus he is contagoius to all cats he comes into contact with. Save this cat get him treated, neutered, vacinated and bring him indoors. He needs a better life than your giving him right now.
Has that cat been vaccinated?
If/when you take him to the vet, the doc will probably want to perform blood work. Sometimes you do not get an answer as to why the cat is losing weight so rapidly, but some people find out it is just a phase in their outdoor lives. Food usually helps with that. Other owners may find their outdoor kitties are borderline diabetic or have thyroid problems. Being outside can help burn tons of calories, and lose weight, especially when they're fighting for a female in heat. During that time they lose a lot of weight, and I mean a lot!
So, it could be serious, or it could be not so serious. You may or may not get an answer, but you do want to give it a try.
I'd like to 2nd all of what zodiacqueen had to say. Get this boy to a vet asap and when he's healthy enough get him neutered. It will keep him from fighting, cut down on the chances of him getting a disease or infection, keep him home more, make him friendlier, and prevent him to contribute to the tragedy of kitty overpopulation.
This sounds very serious, and possibly life-threatening! This cat NEEDS a vet ASAP, losing 5 lbs in a week is very bad. That much weight loss in a week can be caused by many things-to name just a few-severe infection, kidney problems, poisoning, cancer, etc. And, if you do a yahoo/google search on weight loss in cats, you'll see a whole lot more. This is such a serious problem that there is nothing you can do at home that will cure him. You should do everything in your power to keep him inside and force feed him water and food. You can find info on how to do this in the health pages located on this page at the top right. It's above recent activity.
If you want to prevent him from fighting with other cats, please get him neutered if you haven't already.