You can disable it. Settings > General > Keyboard "Auto-Correction" on/off.
i'm sure he likes auto-correction in general, just not in that one instance.
to the op, "me" does not auto-correct to "mr." for me, so perhaps your iphone learned this correction somewhere. i would suggest resetting your phone's dictionary.
settings -> general -> reset -> reset keyboard dictionary
i'm sure he likes auto-correction in general, just not in that one instance.