Do you mean he's not saying much when you talk to him, or that you have not communicated with him directly at all since the 24th? If the latter, could you or one of his friends stop over at his residence and check in on him to make sure he's OK? Is there any chance that he wants to break up with you and this is how he does it--by not returning your calls?
If no one has seen or heard from him since then, you should call the police and file a missing persons report ASAP. He may be in trouble and need help.
Gabriel, how's your dad doing?
misskitty, that is super sad and creepy...