Is he surrounded by a full sphere or only a hemisphere?
The spell emergency force sphere has the entry Effect 5-ft.-radius hemisphere of force centered on you, and the spell's description, in part, says that "you create a hemispherical dome of force…. The bottom edge of the dome forms a relatively watertight space if you are standing on a reasonably flat surface."
Thus, even if the caster is not "standing on a reasonably flat surface," the spell still creates a hemisphere that apparently never covers the caster from below. This should render caster that's also flying wholly vulnerable to effects launched from directly underneath the caster, while effects aimed at the caster from underneath but at an angle may see the caster receive some degree of cover.