It appears from your comments that you have been assigned a Public Defender to assist you with your case. I suggest that you email or call them and schedule an appointment to discuss your issues. You are also free to hire an attorney to help you with your case. Many attorneys offer free initial consultations.
The plea agreement will have an expiration date that is set by the prosecutor. Each prosecuting agency also has polices regarding plea agreements, what is offered and when they expire. It is very important that you contact the PD or hire an attorney to assist your son in getting the plea agreement.