Toddler Bed

Ok, so we've moved Riley (our 16 month old) into a toddler "big boy" bed in preparation for the new baby (we needed the crib!). He had no problem going to sleep in the bed, and cracks us up as we watch him in the monitor in the mornings... he gets out of bed, grabs toys, and jumps back in bed grinning the whole time like he's getting away with something sinister. It really is just cute! But two nights in a row (it hasn't even been a week yet) he's fallen out of the bed at night cause he squirms around everywhere in that bed!! Good thing it's low to the ground! Not that we don't laugh at him, cause he always has this drugged, half asleep look that says "What the hell just happened", but I feel bad for him! I mean, should I put a side rail up so that it stops him from falling out? Or just let him fall out and hope that he learns to not fall out? I just don't know... he stayed in last night, so at least that wasn't 3 nights in a row. I'm just worried that he'll eventually hurt himself, since at least one time (not sure about the second time) he fell out on his head (and then flipped over, wrapping himself in his blanket in the process which made me laugh all the harder... I love video monitors!). I know... not good mommy behavior!!