While most of ScubaBoard pushes everyone to Backplate & Wing, as mentioned above it is not for everyone. In fact, the vast majority of recreational divers dive in jacket BCDs. Jacket BCDs will hold him a bit higher in the water on the surface, which some people prefer. Plus, while BP/W are customizable, most jacket BCDs already come with everything a recreational diver might want included (weight pockets, trim pockets, extra pockets for SMB, slate, spare mask, roll-up snorkel, etc.). Once you add all of the options to a BP/W to do everything a jacket BCD can do, the BP/W might well cost twice as much.
One thing I would want to know is how experienced is your son and has he used a BP/W in the past? If not, I would recommend as above that he either try a BP/W before you buy it, or just rent for these 2 trips.
To the OP's question "what would you do?": What I did do was buy my 14 year old son the
Divers Supply Value System Package with the
Scout jacket BCD, and upgraded to the balanced Covert reg and the 3-gauge console. He's now 18 and has over 100 dives on this set-up, and it has been great.
The Scout BCD, specifically, is quite adjustable for size (he's had a large the whole time, and ranged in weight between approximately 180 to 260 [he's plays football, and bulks up significantly for the season]). The Scout BCD has 2x10 lb. quick release weight pockets in front, and 2x5 lb. trim pockets in back, all likely more than enough to hold enough lead for him for cold dives in thick neoprene -- but unlike steel backplates it is pretty light for flying to warm water locations. The built-in pockets are big enough to hold an SMB and spool on one side (no dangly snag-hazards for swim-throughs) and a slate and/or spare mask and/or roll-up snorkel on the other side. Reflective accents are nice also for low light situations. It's really been a great BCD, and we have no regrets and feel no need to change BCDs anytime soon.