I have been reiterating this on this forums, on other blog forums and even the neighbor kids.
The only way to fix his free throws is by completely ignoring mechanics and just let him shoot, A LOT. Screw mechanics, shoot from where he's comfortable and shoot tons. I see this from my neighbor kids a lot, they're more worried about getting the correct form of their shot instead of just concentrating on making the shot. And I tell them to just shoot and not worry about your form.
I did it when I was young and it worked for me (true story).
Just keep shooting Rondo, forget mechanics and just practice a lot. And don't be afraid to get to the line.