Never plead guilty without a trusted lawyer by your side. You may think the event trivial but no criminal conviction really is. If you go alone you'll be at the Court's mercy which you never should count on.
You should have the attorney appointed for your son without a doubt. There is a chance, based on your jurisdiction, that your son could be treated as a juvenile, which is what you want. Your greatest concern should be your son's criminal record at this point in his life. You want to make sure that he maintains no record and an attorney that understands the local law is the best one to assure that your son's interests are protected. What have you got to lose by consulting an attorney?
Plead not guilty at arraignment, and request an attorney be appointed. If one is not appointed than contact an attorney and purchase some consultation time from him/her; the meeting is confidential and he/she may have some ideas of some things that may be available regarding resolution of the matter short of ending up with something on his record. Even if you can't afford to hire an attorney yourselves and will not be appointed one an attorney via a consultation may have some suggestions. In short, if you walk in and plead guilty and go to sentencing your son is getting the same deal, and no worse or better, than if he went to trial and were found guilty. You've got nothing to lose and perhaps everything to gain by at least talking to a lawyer.