Need advice on what may be wrong with my cat?

I wrote yesterday asking for help and the ones that replied were a big help. I took my cat back to the vet this afternoon. She wants to do bloodwork and xrays, but I can`t afford it til next week. She said it may be a good idea then to put him down. About a month ago, his right eye started to swell. It looks like he`s got a ring around his eye. His eye is partially closed, i just started seeing the 3rd eyelid last week and it has clear fluid draining from it. He sounds like he`s congested when he breathes and I think he`s breathing thru his mouth totally now. He`s still eating and drinking some, but not like he used to. He`s 12 years old and I`m not sure what to do. He came to us as a stray when he was 3 and this is the 1st time he`s been sick. Watching him day after day is breaking my heart. Does anybody know what may be wrong with him???