In one of my new projects the main character is raised on a life of crime due to poverty and the lack of any parental figures, save for his older brother, so his profession is that of a thief. Thieves tend to wear tighter clothing for better mobility; however, he lives in an arid climate, and people in arid climates tend to wear loose clothing to manage the heat better. I know in the end it falls to personal opinion, but I'd like his attire to be realistic and make sense so how would any of you go about it? thank you, Scott M.

IMO thieves tend to wear loose clothing to conceal stolen goods ;)

this is where references come in.
just take a look at prince of persia for a good example. and assassins creed. and probably a thousand other things.

Don't worry too much about what you think thieves should wear, just think about where your character comes from and the local style. when I think of an archetypal thief I think shady, but inconspicuous, yet a little on the hip side.