The fastest and most direct way is to ask your son. He knows what the write-up was for and what sanctions were handed down at his disciplinary hearing. If he is able to make his one weekly call from the SHU, that conversation will tell you everything the facility is unlikely to share with you directly.
From the outside, your options for getting information are limited. The facility is not required to notify family members about disciplinary infractions or the sanctions imposed. His disciplinary record is considered personal information. If you call and ask to speak with his counselor or case manager, you may get a general sense of his housing status but detailed information about the specific ticket and what he lost is rarely shared with outside parties.
On visitation in the SHU, the rules vary by facility and by the nature of the infraction. Standard disciplinary segregation typically includes one visit per week, though some serious infractions result in visits being suspended entirely for a period. Phone access is usually limited to one 15-minute call per week. Commissary is restricted to basic necessities only. Mail continues to arrive, which makes letters your most reliable and consistent way to stay connected right now.
The SHU is genuinely hard time. The isolation, the restrictions, the monotony of 23 hours a day in a cell, it wears on people in ways that are difficult to describe from the outside. Consistent mail from family during a SHU stretch matters enormously even when it cannot be reciprocated right away.
Keep writing. Ask him directly what happened when he calls. That is the clearest path to understanding what he is dealing with.