What is the official term used for building MOC models?
By considering a "somehow usual amongst fans" instead of "official" position (see my comment),  I have heard some fans who simply "verbed" the acronym MOC, as in, "I MOC'ed for 3 hours straight yesterday". You could probably do the same with LEGO itself, but that would be frowned upon as LEGO is supposed to be used as an adjective and so on. (When I was a child, my father used "legoter" in French, which was quite clear as to what it meant. I guess you could be "legoing" in English as well).

I'm not saying I do think it's a very good idea, and I haven't even heard that much anyway; so I would tend to agree with Grandpappy and go for a simple "build" (maybe "build something" to imply it's not a set) or even "play" (or more precisely "play with LEGO" if your son happens to sometimes play with something else (I can't fathom why, though)).

So, in short, no real "official" saying, and not even an usual one - especially considering that fans amongst themselves always use LEGO anyway, which means "what did you build recently" is always understood.