You always have a right to proceed pro se. So you can get your son to file his motion for reconsideration himself. If he has no stamps, you can get it from him at the prison and take it to the court house and file it for him. You can also contact the public defender and let him know that your son wants this filed; do this in writing. If the public defender doesn't do it, your so may be able to file a motion under the post conviction relief act (pcra) to get his appellate rights reinstated.
Your motion for reconsideration must be in writing and it must be filed within ten days of sentencing. In a previous post on this blog, I suggested that you make actually have 29 days to file your motion for reconsideration. However, that is not the law and the judge can simply deny the motion as untimely. The idea is that you have 30 days to file an appeal. However, within the thirty day period from the sentencing until that notice of appeal deadline, the sentencing court still has jurisdiction over the sentencing order, because until 30 days passes its not a final order. Therefore, you could file a motion for reconsideration nunc pro tunc and the sentencing judge would still have jurisdiction over the case.