I always took it he was, essentially, a brain inside a robot that they just plastered his face onto. That's why he needs such meager nutritional intake he only needs to maintain his brain and some skin. At the very most all he is is a skull/head. Otherwise his entire body is robotic.
How would he have felt pain when Boddicker stabbed him in the chest?
I believe part 1 he had some internal organs
Before part 2 they probably "upgraded" him and took out some organic material.
Here's a cross section of what people assume robocop looks like on the outside