He is very close to the end, and the math works strongly in his favor.
In the Texas state jail system, inmates who maintain good behavior earn a 15% good time credit. On a 15-month sentence that works out to about 85% of the time actually served, which comes to roughly 12.75 months. With 11 months already in, he is past that threshold if the good time credit is fully intact.
The key variable is whether his time in county jail counts toward the state jail sentence. In Texas, time served in county jail while awaiting disposition of the case is generally credited toward the final sentence. If those 11 months in county are being applied, he should already be within striking distance of his release date or past it depending on exactly how the credit is calculated.
If the county time is credited and his disciplinary record is clean, he could be looking at release within 60 days or less. The exact date depends on how the facility has calculated his credits and whether any good time was lost along the way.
The best way to get a precise answer is to have him ask his case manager for a current time computation sheet. That document shows exactly how his credits have been applied and what the projected release date is. It takes the guesswork out completely and gives you both a real number to count down to.
He is in the home stretch. Encourage him to keep his record clean these last weeks. This is not the time to pick up an incident report.