18 month old. My son and his now (2 and 1/2 year old son) are living with us. At about 18 months, my son took the convertable baby bed and took the front rail off. The boy has been sleeping in it this way ever since.
(Personally), I think your's is now old enough not be pinned in. I agree with the pillows on the floor and leave the bed open. For the last year my grandson has been sleeping in the converted baby bed, I don't think he has ever ended up on the floor. When the little guy gets put in bed, he knows he has to stay there. He may protest, but he doesn't try to get out. It's all about the way they are trained.
Pillows ... that's my thought!
Edit: You're bringing back memories. When my son was 6 months old, he learned to climb. We couldn't keep him in a crib. We were afraid he would fall on the floor climbing over. We couldn't put a roof on the crib like a complete cage? (could we).
We ended up getting a mattress for a normal bed and put it on the floor. If he rolled off, he only had 8 inches to roll. When he wanted to get up, he could. It worked. The plan must have worked well, he's now 30 years old! (and still living with us, oh my!)