At first I thought that it's Star Kid, but the forehead-eye thing sounds more like Guyver with Mark Hamill from 1991 (which also had a sequel). From IMDb:
A young man discovers a mechanical device that merges with his own body, turning him into a cyborg superhero. When strange creatures start appearing, trying to take the device back, he begins to uncover a secret plot to genetically engineer terrifying monsters.
In this film (which is based on a manga series), his weakness is a metal orb in the middle of his forehead:
You can see the hero wearing this mask-like alien unit on his face and transforming into the Guyver here:
The entire movie is currently available on Youtube here, and its sequel is also available here.