Develop the strength/power base, then apply the endurance specifics.
doing a good mass gain program 'till Christmas and eating like hell, while doing 1 short-intervals workout and 1 active recovery day on the erg
switching at Christmas to a rowing-centric regime (2 days short-intervals, 1-day medium intervals, 1 day long row at active recovery pace) with the lifting program dropped to 2 days/week
I agree with this. Make it happen.
Anything more than really about the 60-75s mark is almost 50/50 split anaerobic/aerobic so you know that 2k row is pretty much 80-90% aerobic. Aerobic endurance does however still require gains from strength/power for enduring more powerful strokes much like longer stride rate in running requires strength.
If you replaced rowing for running you'd get pretty much the exact same concepts in this article I wrote:
I think you should go strength/power until you're at the point of increasing diminishing return, then drop down the volume like you are to make slow progress. The focus on more interval specific work... then transition into endurance base. The thing I like about the fast intervals is they develop the ability to pull explosively and maintain high powerful and thus raise lac threshold and develop aerobic characteristics as well. Then you can translate that better into the increased endurance.
Keep in mind he's young so he still has a lot of developing to go. You can't do everything in one season. Honestly, I think it would be better for long term development to focus on rowing those short intervals, and mass gain + strength/power for at least a year if not more. But yeah. Shrug.