15 months is on the early side, but I've known people that needed to move their child to a toddler bed around that time for the same reason. I think once they have done it once, it's unlikely that they will stop climbing out. I'd recommend going to a toddler bed or matress right on the floor - you mentioned his liking to climb and with either of those options, he'd be close enough to the ground that he's not going to hurt himself if he is climbing, jumping, rolling, etc. He is very young to understand and follow instructions not to do those things especially at times when you aren't directly supervising him like in the middle of the night. Be sure the room is safe - secure furniture to the wall and make sure nothing can fall on him if he pulls on it. Also be sure there is nothing he can climb under the window and be sure those have safety devices too. My windows have them built in - only allows windows to open about 2 inches. If he can open the door, be sure to also put a gate in his doorway so he can't get out and wander around the house while you are sleeping. Then go for it and let him sleep in a toddler bed or matress.