So my answer is COMPETITION, be it with dad, be it with age mates.
Yes, competition works great for our DS, too. He practices way harder for a fiddle competition than he does for a violin recital, even though the violin pieces are harder!
A few years ago, when he needed to review all his Suzuki pieces for an upcoming camp, we set up a competition between him and DH (who plays violin OK). DS assigned each song a point value and wrote the song name and point total on a slip and put them in a coffee can. For about a month before camp, whenever either of them had a free moment they would pull out a slip and see if they could play the song from memory. If they could, they got that number of points. They kept track of how many points they accumulated (honor system). Whoever got the most points at the end of the month won $25 gift certificate for a book store. The were neck and neck the whole time and eventually DS won by 1 point at the last minute--funny how that worked. LOL! But there was very little complaining about practice since he really wanted to win!
