Frankly, my bias is weighted heavily towards simply getting him stronger/bigger first, on the notion that certainly at novice to intermediate levels, strength endurance will be a function of maximal strength. Build the engine first, and tune it up later. I'd lean towards building a strength base using something like Rippetoe's Starting Strength ... and then shifting the lifting orientation towards Oly, to focus on developing power. Ramping up erg work as a proportion of overall training effort at that point.
We've got 2 conflicting models even within our own rowing club. Our juniors spent a winter doing strength training at our (quite powerlifting oriented) local CF gym. Their performance in the next season's regattas had jumped sharply - and the kids making the biggest rowing gains were also those with the biggest improvements in deadlifts and kipping pullups.
At the same time, the top masters rowers in the club are nationally competitive, and they're very heavily focused on erg training, and aerobic/movement efficiency gains.
Personally, I think that reflects where they are in their lifecycles as athletes - they've built their strength, and their aerobic base - and the incremental gains will come from movement efficiency now. In contrast, I think a younger rowing athlete should focus on a strength base, use it to efficiently build the aerobic capacity, and only then priorize the "3rd wave" gains from really targetting movement efficiency 1 per-cent-tweaks.
What worries me is my own bias. Just because I'd far prefer to lift weights by almost any protocol than do metcon, doesn't give me license to give my kid garbage advice.