Some sort of ultra-tech (Apocalyptican?) that could, with the flip of switch, allow an armored carapace to perfectly mimic human flesh, would be the ideal solution, IMO, and could spawn from some sort of deep cover plot where normal looking people turn out to be para-demons in disguise, infiltrating various places to get in place for a surprise invasion. The tech gets introduced with these para-demon infiltrators, and Cyborg gets upgraded after some of his last biological surface components are burnt / blasted / damaged, leaving mostly internal stuff (like his brain) organic. Big clunky robot-looking Cyborg, all metal, but with a swarm of nanites, his metal skin can plasticize and he 'turns human' (looking, and feeling, and to superficial physical scans / biometrics, as his machinery feeds false data to machines scanning him) and can have a more or less 'normal' life, although, even when he looks and feels soft and fleshy and human, his body is still insanely durable (and mostly inorganic)...
Any time when he's expositing, he can transform his face, and only his face, or even cover part of his organic-looking face with a translucent armored faceplate that is barely visible, if he wants to keep his 'face' visible during a firefight, for some reason (to convince someone of his essential humanity, or reassure civilians / hostages / disaster survivors who might be freaked out by a heavily armed 'robot terminator' walking up to them).