My understanding has been that the juvenile charges are always seeled once they are 18.
I remember being concerned about stuff like this when I my AD was arrested for the first time too. Now, I realize that's the least of her worries. She needs to survive her addiction. the longest she ever held a job was a few months and that job was 'exotic dancer' where they don't check weather or not you have a record.
If he's not an addict, he will grow up and stop getting into trouble and this will all be put behind him. If he is an addict, he has a life-long life and death battle in front of him.
Maybe his arrest will allow him to get treament and maybe that will be the best thing that could happen.