If you could get him rehomed in a loving environment then that would obviously be the best option.
Could you post something on facebook, etc ?
I guess you can just tell people that you have a terminal illness....
Perhaps you might want to do that ahead of time, and if the rehoming doesn't work out, then you might decide on the option below.
You could also try to rehome via a shelter, but retain the option of euthanasia if the rehoming doesn't succeed within a certain time period.
Euthanasia is also a viable option in my view, because :
> It will be painless and peaceful, plus they'll be spared the unhapiness of having to miss you.
> They'll be spared future suffering of ill health and cat old age.
> You can both cross over together into the afterlife.....
> A cat is a spiritual being, and that being would understand that you needed to do what you did, and that you took the best action for both of you.....
If you went that route then I suppose you would just tell the vet you have a terminal illness.