A light grey suit is not formal night attire. Of course you can get away with being presentable but formal means a tux and black tie. I will order a tux with a black tie and a cumerbund.
You can arrange for one on board the ship at a price of $85 and you can even call in and reserve over the phone (check www.rccl.com
for the phone number). If you telephone in youur reservation then you need accurate measurements.
I hope this has been of help.