K.W.
I hate to say this, but 14 months is very young to be out of a crib--it's not until about 2-3 years where they learn the "Bed boundaries" so it may be very hard for you to teach him this now. There are also safety issues to consider--getting stuck btween the wall and mattress, etc. The American Academy of Pediatrics recommends keeping chilren in a crib until they are 35 inches tall--almost 3 feet which is usually closer to 3 years of age. I would consider a couple of different options other than trying to keep him in a bed with rails....
1. Putting just the mattress on the floor--this won't keep him in the bed, but will avoid any safety issues. If he is old enough to understand he needs to stay on the mattress, then at least you don't have to worry about him falling or getting stuck.
2. Putting him back in a crib. Maybe borrowing one from a friend or getting one second-hand until he is old enough to understand how to stay in a bed. The crib is really the safest place for him to be and transitioning him at an older age will be soooo much easier. We transitioned my son at 3 and had very little problems or issues.