Restricted housing is solitary confinement, known inside as the hole. It means he was cited for a rule violation and pulled out of general population as a disciplinary measure. The facility is not going to tell you much beyond confirming he is there, which is frustrating, but it is standard.
How long it lasts depends entirely on what he did. There is no universal cap on restricted housing time. Some inmates serve a few days for minor infractions. Others spend weeks or months there for serious violations like fighting, possession of contraband, or having a cellphone. The incident report, which documents the specific charge, is what drives the timeline.
In most facilities, inmates in restricted housing can still send and receive letters, though phone access is heavily curtailed. He may be limited to one 15-minute call per week, or phone privileges may be suspended entirely for now. Start writing him every day. Beyond keeping the connection alive, it will give you a practical read on whether his mail is moving normally. If letters start coming back or go unanswered for an unusually long stretch, that is a sign his mail may be on hold, which typically only happens in more serious situations.
Call the facility again and ask specifically what is in the incident report. They may not share details, but it is worth asking. If you can find out the charge, InmateAid can help you understand what it typically means for his timeline and what to expect next.