My cat is refusing to eat, vomiting, and being lethargic. what could the cause be?

yesterday my cat was vomiting up a white milky substance (we have never fed him milk btw) , lacked coordination and has been falling off of dressers and stopped covering his poop when he uses the litter box. today he is too weak to jump onto the bed and is generally acting weird compared to his normal behaviour. he also hasn't been eating since yesterday. my cat is 15 years old and is skinny and other than these symptoms I can't find anything else wrong when I checked his other signs (gums, ear temperature, etc). I realize that it might just be old age but if there's a way I can help him since we dont have enough money to take him to the vet.