This is my 13 year old baby boy, Han Solo

I adopted him and his sister Cinderella back from the Humane Society when I was 23. He's the most vocal, funny and food crazy boy, but unfortunately he's taken a very sudden turn from lively, to not eating, lethargic and I have to put him on a fluid drop. The most heartbreaking thing, I cannot even afford the testing needed to find out what's wrong or what happened, or if it's even contagious to my other cats. I just got a quote on what it would be to do a blood and urine test, and this amount I'm trying to raise would just cover that. I'm keeping him cozy and comfy at home, but he's not eating and he can't really walk around much anymore. If you are able to help, and if I can raise this, I'll take him immediately to the hospital again.