Just give him some playtime and just keep doing what he likes. If he's really important for you, you should take a look at which players and staff members he likes. Try to sign them and he'll be happier.
at player information it says (for some players) their favorite club and people, some of which can be staff or other players. signing one or two might help. also at interaction you can ask the player to recommend staff or players, and signing those might help.