MrLang, thanks for the reassurance! I thought that it would be normal to reject, but after three in sequence I figured it would be worth posting on here. I learned about 12 things that I can do to increase his happiness in the process, so it was well worth it!

I think I'm going to switch to feeding him once every 10 days (I'll start back with the smaller rats at first).

Underneath the coconut fibre I have a lining of newspaper. During my initial temperature control tests, I noticed that the fibre was conducting the heat too well, and I lined the bottom with the paper - it also helps when replacing the substrate, I've found. I'll make sure to cover it back up so he doesn't try to slither his way in between the paper haha.