coaching isn't going to help him from being too bulky for the SF position. Damien Inglis is enormous for a SF but hes still 10 lbs lighter than Randle and Inglis moves and dribbles better. check out the "lateral quickness" portion of their DX vids and compare, clearly Randle has a more a lumbering step.
Lebron has played PF for most of the season meaning that Julius Randle is 10lbs heavier than any SF currently getting any sort of minutes, the 240lb SF's are Kevin Durant and Joe Johnson, both of these players have handles like a guard.