You often have one player with fine attributes, that has a serious deficiency in one aspect of his game. 

What is the right area to train  him in order to improve?

Based on SI's Mark Vaughan,

 
Strength
Natural Fitness, Stamina, Strength, Work Rate,
Aerobic
Acceleration, Agility, Balance, Jumping, Pace, Reflexes
Goalkeeping
Aerial Ability, Handling, Kicking, Throwing, One on Ones
Tactics
Anticipation, Decisions, Positioning, Movement, Teamwork, Command of Area
Ball Control
Dribbling, First Touch, Technique, Flair, Heading
Defending
Tackling, Marking, Concentration
Attacking
Passing, Vision
Shooting
Finishing, Long Shots, Composure
Set Pieces
Crossing, Corners, Free Kicks, Penalties, Long Throws