I am a law school graduate. What I offer is mere information, not to be construed as forming attorney client relationship.
Yes, it will make a big difference of your son turning 18. He will now have another warrant issued for his arrest and tried as an adult.
You better have a good attorney to get him out of running away from a "marijuana charge", a minor crime in relation to most crimes from which most defendants run.
do a couple of things:
1) read all the material on criminal defense at freeadvice.com's home page AND
2)try attorneypages.com. Look for an attorney in your state who specializes in criminal defense. Some offer free consultations.
3) try your STATE BAR ASSOCIATION AND ASK FOR A REFERRAL FOR A CRIMINAL DEFENSE ATTORNEY.
4) CALL THE PUBLIC DEFENDER's office or DISTRICT ATTORNEY's office and ask about the direct law, WITHOUT GIVING OUT YOUR PERSONAL IDENTIFYING INFORMATION (so avoid telephone lines that will display your name, etc. on a caller id system.)
Hope this helps.