As has been said this sounds like a begging call. All young birds will do this, but with considerable variation in expectation and duration. Just like some young tits never have what it takes to leave the nest hole and stay inside and starve to death, it seems here we have an individual that wants as much attention from its parents that it can get. Evolutionary variation that leads to changes by natural selection in some sense I suppose.
He should get the message that it's not working that well and give up in a day or two!