if he does not have a valid reason for not doing the community service he may be sentenced to a jail alternative, the court will not accept excuses... nor 'i dont want to' so he'd better get himself in gear or face imprisonment.
Not knowing the conditions he agreed to when he plead, it is difficult to answer this. A judge usually sets some form of alternative jail sentence if the person fails to complete it. It would be a violation of his conditional discharge (if he was sentenced to one). With a young individual normally the court will give him a stern talking to and a second chance to complete the community service but if the defendant continues to refuse or false to do it, the judge will sentence him in accordance with the promised alternative.