The erectile tissue of the glans is not covered by a tunica like the erectile tissue in the corpora cavernosa (CC). As the CC fill with blood, they expand to the limit of the tunica and get harder because of the pressure being contained. Since the glans does not contain pressure, it just swells then returns to normal and in many guys never really gets hard.
Kegel’s exercise can strengthen the bulbocavernosus (BC) muscle (see this) which helps pump blood into the glans. Other than Kegels, there’s not much to be done to make the glans harder.