I wouldn't put it in the 55. If it is sick it could make the others sick too.
Test the water in the 14g. If there is something off that is a "simple" fix (salinity, temp, nitrates, nitrites, Etc) it should be easy to correct it.
If everything tests normal and you want to move him.
I'd get another tank (a simple 10g or even a 5g bucket would work) fill the tank/bucket with water from the 55. Acclimate the guy to that water, let him hang out there and see if he does better.