I had a similiar problem with my small just turned 3 year old, and tried a variety of items before coming to a good solution.
Aerobeds makes a nice airmattress with special sides on it for infants. (Aero bed for kids). Well I was so excited about purchasing it but couldn't find it anywhere so asked for some opinions on a forum I am on. A woman wrote to me and told me that PVC can have off gassing issues that can cause illness and even death in infants ~ and the aero bed is made with PVC. I researched it and found evidence supporting her claim and began looking for a new solution.
I ended up purchasing the Regalo my cot (Amazon.com was cheapest) . It is a cot that folds up nice and small, and when set up it is only 7 inches off the ground. My daughter loves it so much that she even wants to use it at home. The first few times of use I put a padded blanket on the floor just in case she rolled off, but she didn't and she then transfered to a big girl bed at home a few weeks later.
If he had trouble sleeping on it, you might be able to find rails that could work with it.
Some other great things about the Regalo my cot... I ended up getting one for each of my kids. Since it holds them up till 52" long and 100lbs, we can use them for quite a while. My youngest is only 16 mo old, but she might be ready to give it a try around Christmas.
The Regalo my cot comes with a fitted sheet for the top, but I also made my own waterproof sheet and sleeping bags for them to sleep in (oldest daughter now, youngest daughter in the next few months).
Hope this is helpful to you!
A.