He was built as a robot in the first place

Maybe he was once human boy and gotten killed in the process by the outbreak or an incident caused by biological weapons?

No It is said he was a robot in the first place Then how would you explain Protoboy and Robotgirl? In a flashback it is clearly shown that Moshimo BUILT Protoboy and didn't turn him into a cyborg

Btw if RB was a human in the first place, he wouldn't be able to be deactivated His human mind wouldn't fit into the small head