Unfortunately he has no chance. Unless Perry runs against him, or if he runs against a very gaffe candidate.
He does have a better chance in 2018 or 2022 though, but it will be tossup at best at that point, not a guarantee victory for him.
Castro would be better off running for a House seat like his brother did or become an Obama cabinet member
Sounds best at this point.
I think you're being a tad pessimistic. If a prominent Democrat with a Spanish surname and a decent record could rack up impressive numbers in the "del" in 2002, it's possible that the same results could be replicated amongst urban Latinos. The real slumbering demographic in Texas are the relatively new Mexican-American communities in Houston and Dallas. If Castro could achieve turnout at the same level as in 2012 and overperform Obama by 5-10%, he could get 45-46% of the vote.
That sounds promising, if the national GOP doesn't take note of Texas falling off their hands, but I think they'll try to get the urban Latinos fast, if that happens.