Him being placed back into custody could be the result of a probation violation. Usually people on probation will have a hearing to show cause to the court why they should not be punished for violating their probation. But if he hasn't had this hearing yet, he could be in custody and awaiting the hearing to happen.
A probation sanction may require him to serve a certain amount of time in the jail or federal detention center and once this is served, he will may be released back to probation. However, the judge could also revoke his probation and he could be sent back to federal prison to serve out the reminder of his prison sentence.
As already stated, violation of any conditions of probation can result in revocation of the probation and a return to prison. That violation would ultimately need to be proven at a hearing to show cause, at which your boyfriend is entitled to have cousel present.
The answer about what will happen may depend in large measure on your boyfriends relationship with his probation officer. If they get along, it may be that a brief incarceration will be sufficient punishment for a relatively minor violation. If they don't, or if alcohol use was thought be the judge or US Probation to be a particularly important provision of the probation, the consequences could be more serious.
Typically, a detention hearing will be held to determine whether he can remain on recognizance awaiting a revocation hearing. The district court will set a court date and the p.o. will submit the person's PSR to the court again with a recommended guideline range based on the violation. I haven't seen a scenario where someone remains incarcerated for an extended period of time for just drinking without a new criminal charge. But, I am not in your district in Washington; however, the guidelines were created supposedly for continuity throughout the U.S.
It depends on the Federal Bureau of Prisons (BOP). A person remains in BOP custody while at a half-way house or "residential re-entry center," and his or her term of supervised release does not begin until he or she is released from the RRC. Therefore, any violations that occur while at the RRC will be handled by BOP, while any violations that occur while on supervised release will be handled either "informally" by the US Probation Office or "formally" by the United States District Court.