a person who is finally convicted of a felony is not eligible to register to vote. Pursuant to Section 11.002 of the Texas Election Code, once a felon has successfully completed his or her punishment, including any term of incarceration, parole, supervision, period of probation, or has been pardoned, then that person is immediately eligible to register to vote.
http://www.sos.state.tx.us/elections/laws/effects.shtml
I did not know that, neither does my son.