Go to a music store, they can evaluate him and make sure he has the right size.
We worked with Dallas Strings with daughter's violin and they are fantastic.. I am not sure if they have guitars. I know they do rent to schools in McKinney and Allen for Orchestra, etc. We ended up purchasing daughter's violin there and we've been very happy. I would at least rent first to know what his interest level is. A good instrument is pricey.. we bought a very good middle range violin at $3000. Believe me, you can tell the difference if you hear this violin vs a rental, a cheap version and vs a more expensive one. Dallas Strings is in Allen.
We also worked with Williamson Music in Plano when she first started out with a rental violin. They did have a lot more instruments and were easy to work with regarding the rentals. Williamson also rents to a lot of the schools in the area.
Also, check with the music teacher at his elementary school. Sometimes they know a good place to get something, they can evaluate on the size needed and sometimes they know if someone has what you are looking for... for sale.
Good for your son, I hope he enjoys guitar.