My 18 year old son often goes for a day or two without any food intake. a) what is it that makes him do this; b) what harm is he causing his body?

Unusual : May there be issues with depression? This is not uncommon and can certainly cause decreased appetite in a teenager.

My son is not eating: I think your son may have some issues that need to be discussed with someone he trusts. I worry about anorexia; i worry about drugs; i worry about depression. Your job is to support him; tell him what you see, and offer to help him find someone to talk to.