Just to give another update on my dad's situation.
Sometimes he can't see how lucky he is to be still alive. All of the examples I gave him in the last years who were still living after a somewhat similar diagnosis as he have past away in the meantime, but my father is still there. I put this mainly here to give others hope that no matter how dark it gets, the sun can somehow shine again one day. Of course I realize this isn't for everyone.
After the devastating news that the cancer has returned aroud the original place in his rectum we knew it was a longshot that a curative surgery could be performed. However a couple months after this news - and during chemoradiotherapy spending weeks in the hospital without the possibility to eat - we've heard last Monday that he can be operated in a few weeks. Scans show the tumor consists of necrosis (dead tissue) for as much as scans are able to show this. Considering that he has bad luck the tumor has returned, he is lucky that it has popped up at this precise place. There will be complications along the way after the operation, however there is a new chance he will have some more healthy years. I can't imagine he will be completely cured one day, but he is living 3 years with the cancer now with good qol and hopefully we can add some more to that after this 6th operation.