can anyone help me make my cali king more comfortable?

[pumpkincrumb]

Hey guys!!! ive only made an account for this thread as i need some advice, so im sorry if i dont return and join any more threads. (also sorry if i post this wrong!!! im doing my best!!!)
So, I have a 5'6"~ (167 cm) 16 yr old California king, and im having some issues with him often being in food mode.
Unfortunately, he's in a 40 gallon terrarium, which i know is very small for him, but that terrarium is from the previous owner, and i havent been able to find a 75 gallon tank to buy. Now that I'm moving, all that i can do is make him as comfortable as possible until i can get him a new one.
But, my snake keeps biting himself. I know it's because his tank is too small, but im wondering if theres a way i can make him more comfortable? I really hate seeing him in pain.
He's also starting to develop some minor scale rot. Any way i can help him with that?
(i feed him a large mouse every week, by the way. ive tried rats because i wasnt sure if that was a good idea, but he just got chunky.)