SID stands for Special Investigations Division, which is the internal affairs unit within the New Jersey Department of Corrections. When an inmate is flagged for a SID investigation, it means something serious enough has occurred or been alleged that it requires a formal investigation beyond what the regular disciplinary process handles. This is not a routine write-up. SID gets involved when the situation involves potential criminal conduct, serious security threats, gang activity, contraband operations, or other matters that rise above the standard disciplinary tier.
During an active SID investigation, the inmate is almost always moved to the SHU, which is the Special Housing Unit or the hole, and placed in isolation while the investigation is conducted. Visits are suspended during this period because the integrity of the investigation requires limiting outside contact. Phone calls may also be restricted or monitored more closely than usual. Mail typically continues to reach inmates in the SHU, which makes letters your most reliable communication channel right now.
The suspension of visits is not a punishment in itself. It is a precaution the facility takes to ensure that whatever is being investigated cannot be compromised by outside communication during the process.
How long an investigation takes depends on the complexity of the matter being investigated. It could be a few weeks or it could be several months. Your son should be informed at some point of the nature of the investigation and the outcome once a finding is reached.
The most direct way to get any information is to contact the facility and ask to speak with his case manager, though they are unlikely to share details about an active investigation. Sending a letter now keeps the connection open while you wait for the process to play out.