I don't have experience with giving sub-Q fluids, although I am following the thread and wishing you and Cosmo all the best.

My own feeling is that it is much to early to feel that you are keeping him alive unfairly. All the evidence on this thread points to the fact that cats can recover very well from this stage and have a good quality of life for quite some time afterwards. If I was in your position, I think I would give it a bit longer and just see how he gets on over the next few week.

Lots of luck to you both.