I need help finding my Mexican Ancestors

I have a Grandfather who was born in Texas in 1923 to Mexican parents.  I'm having a lot of trouble finding any of his family.  He's still alive and so I can't give his name, except that he's a Mendez.  He told me his father's name was Macedonio Mendez and he had a large ranch in San Marcos, Texas.  He never knew his father and was raised by his mother and grandparents.  They were Lara's.  He seems to want me to find out about his family.  I wish that I could find out more about them so I can tell him while he's still alive.  Any advice?