I had written a longer version of this post, but managed to destroy it somehow.
The upshot is, yesterday I think i heard G. say an original sentence-type of thing. He can memorize sections of Bob the Builder episodes and reel them off with impressive feeling, but otherwise he is very stubborn about sticking to the shortest phrase possible.
After a trip to the pool, when he thought I was trying to sneak back out the door (I was actually trying to gather the pile of swim noodles and inflatable pool things we'd dropped in the driveway), G grabbed my arm and said, "Come together...(long pause)...IN!"
So, unless he's been listening to the Beatles, he made a sentence!