Mouth breathing is usually not a good thing. The key is warmth: you must keep the little guy above 80, if not at 85 to 90, all day and most of the night. Most respiratory infections can be beaten this way if still in the early stages. In addition to a warm light, a UVB light really helps as well, especially now that we are getting into the colder and gloomier months. Make sure to combine this heat with the appropriate amount of moisture so the little guy does not dry out.
In addition, nutrition is key. New hatchlings will respond more to squirmy things than fruits or vegetables right now, yet you should place a variety of foods in his enclosure. You should dust the food with reptile calcium powder, as well as spray it with the liquid vitamin/electrolite mixtures that are sold. This will ensure that there are no outright deficiencies that could hinder his recovery. Small slugs, worms, or soft baby crickets will probably entice the little guy right now more than anything else.
I am questioning the wisdom of someone who sent you a hatchling at such an early stage. Could he or she not have waited a week or so to ship? It is getting colder at night, and even during the day, in most parts of the country, and it seems silly to ship a delicate hatchling in these conditions. A couple of weeks would have produced a stronger specimen. Good luck!