This child is not paying rent and is on the verge of causing a separation between the couple with whom he's staying. Does he have to be evicted if he isn't paying rent? I don't know if he has any specific rights since he's been staying there for a few months? Can they just ask him to leave?

One thing you may have to remember
Is that an 18 year old is an adult, not a child.
Hope you find the answers you need

Who cares, if he has rights if he is a adult child and not paying rent, if he is casing problems with the couple, pack up his clothes , set them on the porch and tell him to find somewhere else to live.

First he is not a tenant since he does not pay rent, and he is not a guest,
In most areas the do not fall under any law

First what can happen if he had a right, you get sued for a month rent ? Since his rent is nothing, not much he can win