My son does the same, except he says he is, "just playing" and looks confused when I don't figure it out immediately on my own.
Most of the time I do figure it out, luckily.
I had no idea that what he was doing was echolalia until I hit these boards. (It did not even come up during diagnosis) I just thought it was part of the whole idiosyncratic language thing and that a lot of the stuff that wasn't, I thought was original speech. Every now and then he would use a very out of place $ 5 sentence, and I would remember where it came from, but did not understand what he was doing.
Once I learned about echolalia, it was like an epiphany, and I really understood what was going on.