What is the ruling on one who sees his mother's body when he is 18 years old? Is this forbidden?

A son is not permitted to see more of his mother's body than what is visible to a mahram, such as the extremities. The mother must cover everything else. She must cover her entire body if she fears fitnah (temptation/strife). The reason a son might look at his mother's body could be a neglect of proper upbringing. Therefore, children must be raised to seek permission, lower their gaze, and observe modesty, as Allah Almighty has commanded.