Oh, yes...I see how I messed up! Sorry!
There's still a way for you to get at these records and that is to look at the county records in which the adoption took place. The birth certificate will not be on file with the state and probably not even on file with the county because TX did not start gathering/reporting births this early. Even if Ernesto was born on a reservation the laws of TX would still apply and the adoption would have to take place through the county courts. This would happen through the probate courts in the subdivision of the guardianship or adoption courts. Adoptions procedures (or even probate court procedures) weren't solidified back then so it is questionable which court would have handled this for each particular county so you'd have to do some investigation for that particular county. I think it would also be questionable as to whether or not the adoption would be closed in that period because no one was watching or diligently protecting such records. I doubt very, very seriously if the file on Ernesto -- where ever it is -- is closed. Probate files are public record and anyone can look at them. The only trick to this, of course, is knowing the correct county and the name on the probate file, which could be either Ernesto's or, if his parents were deceased, then the name of the parent who died. It does not matter whether or not the person had a will or even anything to pass on -- if there was a minor child or minor children then by law the Texas courts had an obligation to oversee the guardianship and safekeeping of the minor. The fact that Ernesto's adoptive parents were born in Mexico may have had some play in the procedure, but if their intention was to become American citizens (and most were) then that would have been quashed. Look at the census to see what their citizenship status was. If it says :"Pa" then the first papers initiating naturalization had been filed. If it says "Na" then that means naturalization completed and they went through the courts as well to denounce their former allegiance to Mexico and swear allegiance to the U.S. You will, again, find the first papers and naturalization documentation at the county level -- and it may have been 2 different counties. Naturalizations took place at the county in the District Courts. All these courts sound confusing, and it is until the county is identified and investigated to see how it operated in that period. Its tricky but not rocket science. Please note that after 1906 the procedures for naturalization changed over to the Bureau of Immigration and Naturalization and you can find that index/procedure on the web at
https://familysearch.org/search/collection/1389983 and
http://www.uscis.gov/history-and-genealogy/genealogy/searchi...Do know that 99.99% of people born in TX prior to 1903 do not have birth certificates and a significant number of those born up to, say 10 years later, did not either because the counties were slow to comply with the new ruling. It is possible there was a delayed birth certificate but that could have been made in the name of Ernesto's adoptive parents. A delayed birth certificate did not necessarily have to be filed in the county of birth.
I hope this helps. If it does not then I suggest you contact the Native American tribal reservation in question (if there is one) and talk to them about it. There are only 2 in Texas but it would be surprising to me that a tribe would allow one of its members to be adopted outside the tribal unit...although I can't say for sure.
That's my best try! Good luck.