So, you have to figure out if he's falling out of bed while he's sleeping or when he's trying to get out of bed. If it's while he's sleeping, I think long term the bed rails are the best idea -- at that age you don't want to risk broken bones or sprains. Many of the bed rail designs are easy enough to put down from the outside, or he could just pull back the covers and scoot off the end of the bed. Some bedrails can be placed just near the head of the bed so that he can actually get out around them. If he's falling out of bed when he's trying to get out eg. dizziness (from changes in blood pressure as he goes from laying down to getting up), or lack of muscle control in a drowsy state, etc. you could think about a grab bar, or even a walker placed by the bed for him to steady himself. If it weren't for the memory issues, training himself to get up slower, take a pause as he's getting up, would be very helpful.
As for the vacation, if he's otherwise ok, I think you should go ahead with it. It will be good memories for all of you, not just him, and you may really regret not taking the opportunity if he can't travel when he gets older. Most medical supply stores have rentals available, so you could probably arrange to rent bedrails, etc. and have them delivered to the resort.
One other thing I wanted to mention, you may want to check that his medications are not causing any of these problems. He should definitely be referred to a gerentologist to see if some of these issues can be resolved either by changing medications, changing his routines, etc.