Many times there are glitches in the Judgments of Conviction sent to the Records offices of the correctional institutions by the courts that do the sentencing. Sometimes it's as simple as ordering costs by default that the judge never ordered, and some times it's obnoxious like this situation.
My first recommendation would be to have your son talk to the correctional institution's social worker or records person internally; they're usually pretty willing to explain how they came up with the release date. See if he can look at the Judgment of Conviction that was sent to the institution to confirm it's accurate. If it isn't, you might want to look into hiring counsel who can get a copy of the sentencing transcript and confirm that the judge did say that the sentence was to be concurrent and request that the court amend the Judgment of Conviction accordingly.
There may be many reasons for the different calculation. I would recommend contacting the sentencing attorney and have your son contact the records department at Dodge Correctional if he is still there. Get documentation from the institution and have the attorney explain it. Q