Help pay for my cat's vet bill

Recently as some of you may know my cat, Mr. Whiskers has been in and out of the vet. We finally got to a vet that is willing to run a bunch of tests on him to find out what's going on with my baby. But it's going to cost $1500, I'm only asking for $500, I will somehow come up with the rest using my own money. I just need a little more help.

A little backstory for those who care to read;
Back around the second week of December I came back home from vacation to see my cat has not been eating or drinking and ended up taking him to an emergency appointment at my vet. They believe he might have kidney issues or a tumor. He's 15 so I know he's an older cat but I've been with him since the day he was born, this cat has been with me through it all. He's my lifeline. I need to do whatever it takes to help him. I just need to know how much longer I have with him, or if there is anything I can do to help him.

He wasn't sickly by any means before this, so I believe he has at least a few more years left with me. I need to do something, I need to know what's wrong. I've already dropped $800 of my own money but living paycheck to paycheck is hard and with a bill of $1500 to run the tests they need to figure out what's going on, it feels impossible. Please share or donate as much as you can. I just want to help him.

Thank you!