Myles actually lost in the 3rd round to Jeremy Chardy (eventual winner) so I think Andy Murray was the last in the US Open 2004 (which he won of course!) - though I am not 100% sure as I don't have records of most of the Junior Grand Slams other than Wimbledon.
spoke briefly to dan, he said that Thacher is a gutsy player and has a good serve, However he (Dan) was better from the back and kinda knew how to win better than Thacher does