Actually, he can. He can sue for malicious prosecution, though in Texas state courts it is tough due to restrictions on suing governmental entities. The best way to do it is to allege civil rights violations and sue under those, with added causes of action, in either Federal or state court. Federal is usually the best bet as they are more familiar with the issues involved.
Obviously not your practice area. Re-read 42 U.S.C. 1983
http://www.law.cornell.edu/uscode/html/uscode42/usc_sec_42_00001983----000-.html
And then take another look at Imbler v. Pachtman, 424 U.S. 409, (1976)
"The common-law rule of immunity is thus well settled. 21 We now must determine whether the same considerations of public policy that underlie the common-law rule likewise countenance absolute immunity under 1983. We think they do. If a prosecutor had only a qualified immunity, the threat of 1983 suits would undermine performance of his duties no less than would the threat of common-law suits for malicious prosecution. A prosecutor is duty bound to exercise his best judgment both in deciding which suits to bring and in conducting them in court. The public trust of the prosecutor's office would suffer if he were constrained in making every decision by the consequences in terms of his own potential liability in a suit for damages...We agree with the Court of Appeals that respondent's activities were intimately associated with the judicial phase of the criminal process, and thus were functions to which the reasons for absolute immunity apply with full force."
http://caselaw.lp.findlaw.com/cgi-bin/getcase.pl?court=us&vol=424&invol=409