S.K.
M.,
If you really want him to stay in a crib longer, there are dome shaped nets you can purchase that fit over the top of the crib. eBay would be a good place to look or any baby specialty store would have them.
I put my first two kids in a bed at age 17 months. It took 3 days to a week for them to stop sleeping in front of the bedroom door and sleep in their beds. I went in after they were asleep and put them back in bed so they learned to wake up in the right place. To keep them in bed, you put away things of interest they can reach (books, toys, etc.) and shut the door so they can't get out. If your son knows how to open doors, put a lock on his. If you are concerned about safety, take the lock off after he is asleep. With my third son, I waited until he was 22 months to make the switch and he stayed in bed the first night.
Best of luck,
S.