Would it immediately be more difficult to breathe? Would the air molecules affect him differently? Would regular-sized bacteria around him effect him more harshly? Would everyday sounds become painful to his inner-ear?
Immediately: he would not be able to breathe (normal size oxygen is not good). By the same token, air pressure is now a billion times smaller - he's in high vacuum, and dies of decompression (with miniaturized oxygen and nitrogen boiling out of the blood and causing embolisms) within a few seconds. If he instinctively tries to hold his breath, he'll die much faster of a pulmonar rupture.
Immediately: he can no longer see. His retina is now sensitive to a real-world wavelength 2744 times shorter, at the border with X rays. Without an X-ray source to illuminate his surroundings, he's in the dark. Conversely, the infrared radiation he emits is now 2744 times more energetic and becomes hard ultraviolet; he becomes a millimeter-sized sterilizing lamp.
Almost immediately: his surroundings should feel "sticky", with Van der Waals forces anywhere from 2744 to 7.5 million times stronger depending on atomic interactions.
In a few moments: his body would continue generating the same heat as before, about 100-200W. But now he has to shed it from a surface which is thousands of times smaller. If his height is reduced by a factor of 143, his surface is now almost eight million times smaller, and its equilibrium temperature must increase by the fourth root of 7.5M, from around 300K to 15600K - which is around 15330 °C. Or more, since that formula accounts for radiative cooling while a human body is mainly cooled by atmospheric conduction and evaporation. According to an alternate interpretation of the theory, heat is now unable to escape from his body towards the environment, either from skin or airway water evaporation, which means that he'll quickly cook alive. His body temperature will exceed 43 °C within twenty-thirty minutes, at which point death by hyperpyrexia occurs; decomposition will raise it further somewhat, not that he'll care anymore. The increase in temperature may bring his body radiation into X-rays or even soft gamma range, and instead of a small sunburn his micro-corpse can now bequeath bone cancers.
Possibly: his body surface has a greater atom per square real millimeter ratio of anything nearby, and any electrostatic surface charge now exerts an invincible attraction on matter. He'll quickly be swamped in dust, grit and dirt from the ground unless he uses some kind of antistatic metal jumpsuit with internal capacitors.
Possibly: the electrons emitted by the body have the same charge as a normal electron but have a Coulomb repulsion radius which is now negligible (charge/volume ratio has increased enormously). Most positive ions happening to float nearby therefore undergo catalysed nuclear fusion, adding macroscopic havoc of unknown proportions (possibly insignificant, but I wouldn't bet on it) to the already considerable microscopic mess.