There is the possibility that Thomas joined a local militia group that never transferred to the CSA. I had Texans, one that died before his unit transfered, one that was back home & his did not transfer, while another one joined a local militia and was with that unit when it "officially" transferred to CSA. You might contact the TX State Archives, see if they have anything on Thomas.
Check the Dallas Co, TX GenWeb site, some GenWeb counties are putting their cemeteries online. Also, post a query, see if anyone has a copy of a published county history, and will they check their copy for any possible info on your Thomas and his parents. If they were there by 1848, good chance they will be mentioned. Many county histories will list men who fought in all confrontations, but especially the Civil War. Even if it was a local militia group.