I like your idea -- your reasoning makes sense.
I would also suggest high-hang or block snatch/cleans with the bar set at above knee or mid-thigh. That should help him understand the need to finish his pull up before beginning his pull under the bar. Once he grabs a hold of the concept, work the bar down the legs until the bar starts on the floor.
Another could be doing pulls + lift complexes. It could be something like 2 pulls + 1 snatch/clean. The pulls will help him feel the full extension and help it start becoming muscle memory.
I hope that helps you and your lifter.
Javier A. Sanjuan
Olympus Barbell Club
Dear God, please help me lift heavy and be awesome. Thanks. Amen.