What do French men wear when they vacation in the South of France in
summer? Are shorts OK? Sneakers? What kind of shirts? I don't want to
look like a tourist

Why don't you just wait till you get there to buy what you need?

Have a nice time, Myoarin

If you look like a tourist, you won't be the only one. There is no
usual dress for french men in south of france. Shorts are fine, as are
sneakers. Re shirts: polo shirts for casual activities and take short
sleeved smarter shirts for dining in the evenings, with light
trousers. A jacket (i.e blazer) might also come in handy, but don't
worry. Noone will bat an eyelid if you parade around in a torn old
t-shirt and jeans.
Have a great time: it will be hard not to.

Everytime I've been in France I've followed three rules that have served me well:

1.  No sneakers (unless they're slick and/or retro--think Pumas,
Addidas or Roos), especially the white ones.
2.  No jeans (unless they're dark wash and if you're a man,
accompanied by a nice jacket).
3.  Dress up, rather than down.

I'm not saying I walk around in black Versace gowns; I just try and
look polished and somewhat hip.  I have been mistaken for a French
citizen on a few occasions, presumably due to my dress code (or
perhaps my love of cheese).

That being said, whatever you wear, be it church clothes or jeans and
sneakers, look confident and it'll fly.

And don't forget to say "Bonjour madam/monsieur/mademoiselle", "S'il
vous plait", and "Merci."  That will take you a long way.

Have fun!  Sounds like a great vacation.

white used t shirt
try to find espadrille , typical for frnch people if you go to the beach only
light jeans
no hat
sun glasses needed
http://www.vivacollection.com/men/mensespadrilles/images/viva010.jpg