20 months is quite young to drop the nap entirely, however my kids are not LSN so I'm not speaking from BTDT experience. Would he nap in the stroller if you took him out for a walk? Is he playing hard during the day? If he's expending lots of energy with activity, he'll likely need at least some kind of recharge mid-day. I'm sorry I can't offer greater advice, but I do agree that 12hrs of sleep in 24 seems a little low.