What are my inmates chances of getting a shorter sentence?

My husband got sentenced for 6 months in a state jail in Bartlett Texas. I was wondering if he would be able to get a shorter sentence with good behavior since reading some of these questions. Last I was told is that there is no good time served reduction for Texas state jails (tdcj). This is a 10 yr old case and he's had his fines paid off. If it is possible what can I do to help get him that or is it something only he can do and how? Thanks so much, Niki