I think that
Rainbow Dash and Dorchadas' suggestions are both good. Honestly, I don't know if you even need the X in the real name to get
Tex as a nickname - I could see a
Tennyson called
Tex, or something along those lines. I think I have a bit of a tendency toward unintuitive nicknames, though, so I might be the only one who thinks that that could work.
I think that putting Texas or
Tex in the middle spot and then just calling him by that would work as well.
Interestingly, there were apparently 8 boys named Texas in the U.S. in 2010 and 9 named
Tex.
Also, Tennessee
Thomas from The Like was born in England with the first name of Tennessee, so there's some precedent for U.S. state names used as first names in the U.K.