Thank you all so much for your advice- it helps to hear from people who understand. I'm so sorry to hear of other cats going through what we are.
Odin had a pretty good day yesterday, ate quite a bit and even managed to have a swipe at his sister Freya, like he used to.
It is so painful to see him like he is now, he actually used to be overweight but is just skin and bones now.
We have decided to see how he is at the end of the month, he is still having good days now but they are fewer, most days he just sleeps the whole day through.