Logically in his mind, he feels he's a better player.  Therefore he should get more play time.  Nothing wrong with that logic.  Seniority is a fact of life though as unfair as it is.  I don't like to practice it in the work enironment but there are rare instances when decisions have to be made and length of service becomes a deciding factor.  Teaching him to roll and bow down may not necessarily be the best thing to teach him.  Has he pointed out his feelings to his coach?  The coach might take it into consideration.  The coach might even admire his confidence and try not to discourage it.  Throwing tantrums however won't get him far; a mature approach is best.  Making the coach aware might make the coach give him the encouragement he needs.