Seems to me that many amateur compositions rely rather too much on one single idea.
Right. I think composing a piece out of a single idea is one of greatest challenges in music. Many amateurs go the tough way and try to write slow and lyrical pieces instead of short and joyful ones (which is of course not easy, either).
Not really the case here. In my opinion, your son did a very good job, Tom. Very moody and quite dark, I enjoyed it. And I think the title makes sense. It isn't an invention in the sense of counterpoint, but in a romantic sense: maybe something like "reminiscences of an invention". At least, I hear some passages that imitate the sound of counterpoint a bit.