I would attempt to draw him out and get him to describe the thing he's trying to label. Maybe there is a name for it, and you can help him with that. Maybe there isn't, and he can make up a name for it himself.
Either way, you'd also be helping him develop more flexibility in his language, and that's an important skill. It's not helpful, for instance, to only use the proper terminology if your audience doesn't know what those terms mean, so you have to be able to adapt. I've also found it helpful in expressing myself in foreign languages in which I'm far less than fluent.
