Having charges dismissed, whether for a misdemeanor or a felony, should have no relevance to the disposition of new charges.  In other words, in light of the fact that our justice system presumes innocence until PROVEN guilty, if charges are dismissed then the defendant's presumption of innocence remains.  In any event, shoplifting charges for a minor usually are not that serious.  Often times, they can be resolved via a diversionary program so that there is no conviction.  In any event, in light of the fact that this may be a pattern of behavior on behalf of your son, I suggest that you speak with and/or retain a local criminal defense attorney as soon as possible so that your son can protect his interests going forward.  Good luck.