My 14 year old cat is gravely ill is there any hope for him?

He stayed overnight Thursday at the vet as he was struggling for breath (he has suffered with a breathing problem for a few years now and has regular steroid injections). His breathing is now better than it has been in years but I think he may have cat flu now (goodness knows where he got it as he's an indoor cat) he has had thick green mucus coming from his nose for the last couple of days and he is very very poorly. He continues to drink and wee but is very unsteady on his legs. He has not eaten very much since Friday but he has eaten something and minutes ago, he licked the jelly from some wet food. He just looks so ill and I've to take him back to vet tonight but I'm petrified he will need to be put to sleep as he's my baby, I don't have real children and I'm so scared. Is there any chance of him getting better?