Aside from Celeste back at the beginning, Dean is the only human that’s ever seen Sean. Walt has been keeping a close eye on him ever since they met and acting as a surrogate father for the newly-cursed boy, and if anyone knows what they’re doing, with both scavenging for supplies and raising former humans, it’s Walt.
Even Dean didn’t see him for the first few weeks he was at the motel. They made damn sure that Dean knew what he was doing around people their size before risking anyone new out in the open where they’re so vulnerable.
Brothers Apart 