Graduating from a correctional drug and alcohol treatment program is a significant milestone and in many cases it is directly tied to release, but the exact timing depends on how the program completion interacts with the underlying sentence.
In some systems, particularly for sentences where the treatment program itself was the condition of a suspended or deferred sentence, graduation triggers release processing almost immediately. In others, program completion is a prerequisite for parole consideration rather than an automatic release, meaning the parole board still has to act on the case before he walks out.
The facility took him when a program slot opened rather than on a fixed schedule tied to a release date, which is common for treatment placements. That intake timing sometimes means the graduation date and the release date do not align as neatly as they would on a standard sentence. Administrative processing, bed availability at any halfway house placement, and parole board scheduling can all add days or weeks between graduation and the actual release date.
The most accurate answer comes directly from the facility. Call the correctional treatment center and ask to speak with his counselor. They have visibility into his specific case and can tell you what the out date looks like following his December 22nd graduation. That call takes a few minutes and produces a real answer rather than an estimate.
In the meantime, a letter through InmateAid, celebrating his graduation and letting him know you are ready and waiting, is worth sending. He has done the hard work and the end is close.