Plea bargaining is just like any other sort of negotiation. The more the defense has to trade the better outcome he can negotiate. The best plea offers are made to defendants who have some legal or factual defense to mount if the case is litigated. For example, sometimes a better plea offer is made to a defendant who agrees not to litigate a sound legal motion. Without seeing the police reports in your son's case, it is difficult to know what, if any, defenses or legal motions might have...