Do NOT let him listen to it!!! The mids (and others) who push that are usually not part of that major, so they (especially the mids) really don't know what they're talking about. NOBODY who was in my section at the end regretted doing it and we were probably the most tightly-knit section (only 21 of us in '09). Do not worry about those who say that "the engine stuff will detract from his deck education." We had some of the highest averages in our deck classes and did better on license than the other sections - your deckie stuff will not suffer much, if at all. The best faculty contacts at KP for the shoppers (including those prospective plebes) are CAPT Brown and CDR Gardner, both Dept of Marine Eng. I would encourage your son to talk to them before making his decision, as well as some upperclassmen who are actually shoppers, preferably the first classmen who have already done their engine time.
I should also add that IF he doesn't like it (or can't hack the extra academics), he can always "downgrade" to straight deck, with little consequence, if any. Doing the opposite (ie: starting as straight deck and deciding he wants to be a shopper) is damn near impossible, especially after plebe year.