No, if he is in TDCJ then he has already been sentenced.
In Texas, if you break the law, you start off usually in a county jail. You are there while you have your court hearings, and the sentencing. Then you wait for TDCJ to collect you.
If you have further charges to face after you have entered TDCJ, you usually go back to county jail while you go through those court procedings, and then back to TDCJ to serve the sentence.
If you misbehave while in TDCJ, you normally just have a disciplinary hearing, but if you do something serious like start a fight, hit an officer, etc then some prisons have specific areas of detention and that's all I can think of at the moment that would relate to "DC". It would also help explain why he cannot have visits at this time, if he is under investigation for something. He has probably had his property removed and may not have any writing supplies, so you may not hear from him for a while.
I would suggest sending him a card or postcard every few days, just so he knows you are still there for him, but don't keep telling him you haven't heard anything because if he can't write that will either anoy or depress him. It would be OK to just say something like "I can see on Jpay that your cell assignment has changed, so I'll just keep sending you these cards until you can write to me and let me know what's happening."