From my quick research, the Dark Rome tour includes a trip to the top of Mount Vesuvius. If your son is obsessed with Pompeii, I would think he would enjoy the "full" tour of Pompeii with the view at the top of the volcano - where it all started.
The Walks of Italy Tour does not include the Mount Vesuvius trip; however, it does include a bus ride along Amalfi Coast.
It's all a matter of preference - but if he's obsessed with Pompeii, he probably would enjoy the Mount Vesuvius part better than the Amalfi Coast tour.
I am in the initial stages of planning our trip to Italy. We plan on staying a few nights in the Pompeii area to fully experience Pompeii, Mount Vesuvius, Herculaneum, and the Archeological Museum in Naples. We will most likely use a private guide since we won't need the transfer from Rome.
Have fun on your trip.