Of course he can wield it. In general he would need the proficiency or take the penalty(-4). Then he would also take an improvised weapon penalty unless he took an exotic weapon feat. Yeah, two feats in he primary hand, and of course TWF if in his off hand.
Besides the fact that this is dnd and there are rules; A two handed weapon is a weapon for two hands. They are shaped and balanced differently. It's like asking to use a bow with one hand because your bigger. heh.
On the other hand, what's the beef? Can't you just get a Longsword of one size catagory larger and wield it in one hand?