Is he crawling? Pulling himself up on stuff? Do you carry him all the time, or does he have adequate time to roam around? Can he walk if you're holding his hand?
ETA: Just saw your update, thanks :) If he's dropping as soon as he realizes he's not holding into anything, that seems okay. He's just the little lion from the Wizard of Oz and needs to find his own courage :)
Just to be sure, when he goes in for his 18 month check up, have them check his ears and make sure everything's okay with his inner ear and equilibrium. If he was a little slower than his sisters, that's okay, boys develop most motor skills a little slower. Don't compare kids, you'll create a nightmare for yourself ;) Check with the doc, but I'm pretty sure if you continue to encourage him, it will 'click' and he'll be running around in no time :)