Give the vet a call. Explain to them that you can't afford the fees to bring him in overnight unless its a huge emergency. Don't worry about shame I've done this before, most people think twice about going to the vet, its stupidly expensive! Tell them what's happening and ask them to tell you what you need to look for to say that kitten is getting bad. They should tell you, if they don't call the RSPCA, they might be able to tell you.