If Butner is the facility listed on his sentencing paperwork, that is where he is going. The Bureau of Prisons follows through on designations. Tallahatchie is simply a holding point while the transfer logistics get arranged.
What you will not get is any advance notice of when it happens. That is by design. Transfer dates, times, and routes are kept confidential for security reasons. The facility will not tell you, and your son likely will not know until the morning it happens. Inmates are typically woken early, told to pack their property, and moved out before most of the facility is awake. It is deliberately quiet and quick.
The communication blackout that follows can last anywhere from a few days to a couple of weeks. He will be in transit, then going through intake and processing at Butner, and phone access does not come back until he is fully settled in his new housing assignment. That silence is unsettling but it is normal.
The way most families find out the transfer happened is exactly the way you would hope. One day the phone rings and it is him calling from Butner saying he made it. That call will come.
In the meantime, if you want to send mail, hold off until you have confirmed he has arrived at Butner. Anything sent to Tallahatchie after he leaves will get returned or sit undelivered. Once he calls with his new housing information, InmateAid can get a letter to him at the new facility quickly.