Thank you for trusting your question to JA today. I am a licensed attorney with over a decade of law practice and over 20 years of experience in the legal field. I’m happy to be of assistance.
Getting these things changed (specifically, the RE-code is what you need to change) is exceedingly difficult. It's not that the process itself is hard. It's just that the board rarely grants the request.
Your son would have to file a DD Form 149 with the military records correction board, asking that they change the record to allow him to rejoin. He would have to prove either an error or an injustice to get them to change it.
You can't show error because that is an legally available RE code for this situation, so you'd have to prove injustice. You stated that you feel this is overly harsh. It may have been for his particular unit perhaps, but military wide it is not. It is typical.
However, if he can show that he was unfairly singled out while others received more favorable separations, then he can perhaps convince the board.
If you have any further questions, please let me know. I invite follow up questions, so use REPLY for those. If you have no further questions then good luck going forward and please do not forget to rate my service with a top-three rating so that I receive credit for working with you today. Please rate me based on my service and not on your satisfaction with the law, which I am not in control of and I am just reporting to you. Also, feel free to request me in the future, if you have questions concerning a different matter.