With a 500K serial number it has gone through RECAP, they issue a new serial number and reset the build date, it is actually probably a 2005 or later year model.
NOW if he can get that re-assigned to a 1994 or earlier that would be best but in reality it is no longer a 1980s/90s truck, and in TX it really does not make any difference unless he wants to put the restrictive but cheaper MV plates on it, that requires the vehicle to be a min. of 25 years old IIRC.
+1 on age doesn't matter on FMV registration. My 1123 is registered at FMV as a 2000 year model. 2000 was on my SF97 (govplanet asked what year I wanted on the form). Besides the significantly lower registration cost, the other advantage of FMV registration is that you don't need to have it inspected. And you have the option to run without physical license plates and just use the military stencil markings as your license number.
There are restrictions on how you can use the vehicle and you are supposed to keep it original looking, so probably not a fit for the folks looking to paint their hmmwv a different color and put aftermarket rims on it. Also not a good fit for a daily driver. FMV registration says it's only supposed to be used for parades, club events, exhibitions, and functions of public interest and not for normal transportation. When I'm in my hmmwv, it's pretty much always a exhibition and function of public interest in my opinion, but a cop could certainly question that if he wanted to press it. No issues in the last year and have only gotten thumbs up and waves from law enforcement. Really, my main concern is running without plates because I could see a cop questioning that if they don't know about the option to run with the miltary stencils. But my truck also looks like it could be an active military vehicle, so maybe the original appearance is reducing my chances of getting pulled over for lack of plates. Regardless, if I have any issue with the law, it's supposed to be an easy switch to change the registration once you have the title. I can either add pysical FMV plates to my FMV registration or change the registration to regular truck as long as I get it inspected.