You're right, it's just experience of playing him there. It takes a little longer than it used to but it works. I don't believe the development sticks if you sell him though, think it used to but was changed a year or two ago.
(also worth mentioning that if you use a player in a tournament, it will automatically maximise a player's fielding potential, so Duffy would be a 94 in LF there without any development
)