The guy has been invited to a competition at Legoland. At the same time, he's rediscovered his Duplos and is excited by them! I figured he'd develop a "they're for babies" attitude about them, and so I'm pleasantly surprised.
As much as I love Legos, I have mild concerns about lock-in and associating building exclusively with Lego (TM). I think the attraction to tools instead of concepts and principles is a problem in the larger world.
So, this is lucky, and I have to remember to do my part to encourage heterogenous building.