I bought this banana plant the early part of this year from a specialist garden center - obviously it was a healthy specimen at that point.
Until now i've kept him in a garden shed and over the last month i've tried to harden him off. Initially i bought him inside the house by a set of patio doors which captured the sun - popping him outside on the nicer days. He didnt seem to develop so i popped him in the greenhouse for about 2 nights and the picture shows the result.
His stem has started to go quite soft and looks 'bruised' and any leaves that were developing have browned off. Im quite worried if i leave him he may die off completely. Can anyone suggest what is wrong and a remedy to solve this? I was wondering if to cut the top 'bruised' stem off in the hope for some re-growth.

I live in the UK - and this is the first banana tree I've owned - so a novice. Please help.