Being moved back to prison from a transitional center this close to a release date almost always means something happened that violated the conditions of the placement.
Transitional centers and residential reentry facilities operate under strict rules specifically because the people there are in the final stage before full release. The conditions are more relaxed than prison but the expectations are non-negotiable. A violation of those conditions, whether a failed drug test, missing a curfew, unauthorized contact with someone prohibited by the release terms, a new arrest, or any number of other infractions, can result in immediate removal and return to a more restrictive facility.
The fact that he will not tell you why is itself informative. People who get sent back for reasons they are comfortable explaining generally explain them. Silence on the specific reason usually means either embarrassment about what happened or concern about how you will react to the information.
The most direct way to get clarity is to call the facility he is now housed at and ask to speak with his counselor. If you are listed as a family member or close contact in his file, the counselor may be willing to share general information about the circumstances of the transfer without disclosing specific disciplinary details. They will not always share everything, but the call is worth making.
Whatever happened, the priority now is understanding whether the release date at the end of the year is still intact or whether the violation has pushed it further out. That is the practical question that affects everything else and the counselor is the right person to ask directly.