There is nothing magic about turning age 18 if the case began when he was a juvenile. The punishment may become more severe once he is 18, but the judge he will see will not change based on his age. As other counsel have advised, the best thing to do is retain counsel and turn oneself earlier, rather than later.
He will be taken back to the juvenile court regardless of whether or not it is a day before he turns 18, or a day after. The juvenile court will still have jurisdiction. It is better to contact his attorney, or get one, and surrender himself as soon as possible. It will be better int he long run.
Crystal balling it is very difficult for everyone. The only way to find out is to do it. But I will say this that the longer you wait the more difficult it will be for you so get it done soon.
The response above is not intended as legal advice since it’s impracticable to provide thorough, accurate advice based upon the query without additional details. It is highly recommended that one should seek advice from a criminal defense attorney licensed in your jurisdiction by setting up a confidential meeting. Moreover, this response does not constitute the creation of an attorney-client relationship since this message is not a confidential communication because it was posted on a public website, thereby publicly disclosing the information, which is another reason to setup a confidential meeting with an attorney.