Normally when I see such players in the youth/reserve team I fire them. But seeing you have spend quite a bit on the young lad, there's no other help than fining him when he gets a direct red in youth/reserve matches. That should lower his aggression (and might even raise his decision).
You can discipline the player in reserve games if he gets sent off and his aggression will reduce but you should consider if his aggression compliments his game - what position he plays? is his tackling good?
His Aggression may not be the problem, just a contributing factor. Have any scouts or coaches described him as having a competitive streak that can get him in trouble? If so he will have a high Dirtiness attribute (hidden) which will come into play more often because of his high Aggression.
If not/also, as his Aggression means he will seek to get involved in everything, then low levels in certain attributes may be a problem eg Tackling, Positioning, Anticipation, Decisions, Acc/Pace etc. if one or more of these are particularly low then he could be making too many mistimed challenges leading to injuries.