If the mattress is really thin you could...make a quilt the size of the matress, maybe a wee bit bigger,attach large triangles of fabric to each corner [almost like an old fashioned photo corner holder thingy), on the back to hold it in place so it doesn't move around . That way if the cot ceases to be used the corners could be removed and he could still have his quilt.
Somewhere online I saw a pattern for kids that was folded in half over them similar to a sleeping bag and had a little pillow atatched, maybe a tille pillow might be nice, if you attach it maybe on straps, it would all stay together, I think that with many small children in one place things can get pretty crazy, I think you need easy to use .