This snake should NEVER have been near your other snakes for even a moment, okay? Quarantine ALL new snakes- you never know what they may have that all your others can catch & die from.
Agree with Armiyana's post- IBD or RI are all possibilities, as is just poor health (weak) & genetic issues. Also, many snakes are fascinated by the new scents in the air, so they'll naturally want to stick their nose in the air. I'd re-assess his balance (ability to right himself) in the future, as so far your results were inconclusive. I know how hard it is not to rescue snakes in bad shape when you see them, but it sounds like you took on quite a lot- especially without a good herp vet closer to you- this was perhaps not a good idea. (He's pretty though...I understand your temptation- I do.)
With an underfed snake, I'd not have fed a "large mouse" as that's harder for the snake to digest, so you risked a regurgitation which would have been a huge set-back. Remember that no matter how badly you want to feed a starving snake, it's best to go slow- their digestive enzymes will likely be in short supply due to malnutrition, so they'll have trouble digesting the larger mouse you couldn't resist feeding him. Large mice (as in older breeders) are also very fatty, which again takes longer to digest- it would have been better to have this snake eating frequent smaller meals for quite some time. Growth- & rebuilding a thin snake takes time. And hydration is essential too- if a snake is dehydrated (as this one seems to be based on your description) that also makes digestion difficult. So remember to go slow- it just works better & pays off in the long run.I hope it turns out to be nothing serious.