This Boy Can Fight Aliens

Kono Danshi, Uchuujin to Tatakaemasu.

When aliens suddenly invade the world, Earth seems to be fighting a losing battle until Arikawa, one of the defense command staff, accidentally discovers a young man lying on a hill. Tests soon prove the impossible: this one teenager, Kakashi, has the power to defeat the attackers! Unfortunately, Kakashi has also lost his memory, and with it the knowledge of how to actually use...