My cat is seems to have lost weight. He has also constatly been looking to outside of his eyes. I think he may have a disease called the third eyelid. (look it up). When he sits down his hip bones pop up about an inch.he is only 4. if you know or may know what's wrong please tell me.

There is no way for us to know what is wrong with your cat, but it seems that you really need to get him in to see the vet.
It could be any number of things.
Also "third eyelid" is not a disease, it is a membrane in the eye that you usually do not even see.
If you are seeing it then something is wrong somewhere.
Whether it be illness, infection, dehydration, the list goes on.
Please have him checked.